diff --git a/src/column-layout/flexible-column-layout/index.tsx b/src/column-layout/flexible-column-layout/index.tsx
index c80c38f2d0..9c0fa1c725 100644
--- a/src/column-layout/flexible-column-layout/index.tsx
+++ b/src/column-layout/flexible-column-layout/index.tsx
@@ -1,11 +1,11 @@
 // Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 import React from 'react';
-import flattenChildren from 'react-keyed-flatten-children';
 import clsx from 'clsx';
 
 import { useContainerQuery } from '@cloudscape-design/component-toolkit';
 
+import { flattenChildren } from '../../internal/utils/flatten-children';
 import { InternalColumnLayoutProps } from '../interfaces';
 
 import styles from './styles.css.js';
diff --git a/src/column-layout/grid-column-layout.tsx b/src/column-layout/grid-column-layout.tsx
index 067c725df8..c303e4181d 100644
--- a/src/column-layout/grid-column-layout.tsx
+++ b/src/column-layout/grid-column-layout.tsx
@@ -1,12 +1,12 @@
 // Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 import React from 'react';
-import flattenChildren from 'react-keyed-flatten-children';
 import clsx from 'clsx';
 
 import { GridProps } from '../grid/interfaces';
 import InternalGrid from '../grid/internal';
 import { useContainerBreakpoints } from '../internal/hooks/container-queries';
+import { flattenChildren } from '../internal/utils/flatten-children';
 import { InternalColumnLayoutProps } from './interfaces';
 import { COLUMN_TRIGGERS, ColumnLayoutBreakpoint } from './internal';
 import { repeat } from './util';
diff --git a/src/grid/internal.tsx b/src/grid/internal.tsx
index cc7c05881c..f5893669ec 100644
--- a/src/grid/internal.tsx
+++ b/src/grid/internal.tsx
@@ -1,7 +1,6 @@
 // Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 import React from 'react';
-import flattenChildren from 'react-keyed-flatten-children';
 import clsx, { ClassValue } from 'clsx';
 
 import { useMergeRefs, warnOnce } from '@cloudscape-design/component-toolkit/internal';
@@ -11,6 +10,7 @@ import { Breakpoint, matchBreakpointMapping } from '../internal/breakpoints';
 import { useContainerBreakpoints } from '../internal/hooks/container-queries';
 import { InternalBaseComponentProps } from '../internal/hooks/use-base-component';
 import { isDevelopment } from '../internal/is-development';
+import { flattenChildren } from '../internal/utils/flatten-children';
 import { GridProps } from './interfaces';
 
 import styles from './styles.css.js';
diff --git a/src/internal/utils/__tests__/flatten-children.test.tsx b/src/internal/utils/__tests__/flatten-children.test.tsx
new file mode 100644
index 0000000000..8e4a13a173
--- /dev/null
+++ b/src/internal/utils/__tests__/flatten-children.test.tsx
@@ -0,0 +1,70 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+import React, { Fragment } from 'react';
+
+import { flattenChildren } from '../flatten-children';
+
+describe('flattenChildren', () => {
+  const nestedArrayChildren = [
+    <div key="1">Item 1</div>,
+    [<div key="2">Item 2</div>, <div key="3">Item 3</div>],
+    <div key="4">Item 4</div>,
+  ];
+
+  const fragmentChildren = [
+    <Fragment key="group">
+      <span key="a">A</span>
+      <span key="b">B</span>
+    </Fragment>,
+    <span key="c">C</span>,
+  ];
+
+  const singleFragment = (
+    <Fragment>
+      <span>A</span>
+      <span>B</span>
+    </Fragment>
+  );
+
+  // Tests React 19+ behavior: uses Children.toArray() which does NOT flatten fragments
+  describe('React 19+', () => {
+    beforeEach(() => {
+      // Mock React.version to trigger Children.toArray() code path
+      Object.defineProperty(React, 'version', {
+        value: '19.0.0',
+        writable: true,
+        configurable: true,
+      });
+    });
+
+    it('flattens nested arrays', () => {
+      expect(flattenChildren(nestedArrayChildren)).toHaveLength(4);
+    });
+
+    it('does NOT flatten fragments', () => {
+      expect(flattenChildren(fragmentChildren)).toHaveLength(2);
+      expect(flattenChildren(singleFragment)).toHaveLength(1);
+    });
+  });
+
+  // Tests React 16-18 behavior: uses react-keyed-flatten-children which DOES flatten fragments
+  describe('React 16-18', () => {
+    beforeEach(() => {
+      // Mock React.version to trigger react-keyed-flatten-children code path
+      Object.defineProperty(React, 'version', {
+        value: '18.2.0',
+        writable: true,
+        configurable: true,
+      });
+    });
+
+    it('flattens nested arrays', () => {
+      expect(flattenChildren(nestedArrayChildren)).toHaveLength(4);
+    });
+
+    it('flattens fragments', () => {
+      expect(flattenChildren(fragmentChildren)).toHaveLength(3);
+      expect(flattenChildren(singleFragment)).toHaveLength(2);
+    });
+  });
+});
diff --git a/src/internal/utils/flatten-children.ts b/src/internal/utils/flatten-children.ts
new file mode 100644
index 0000000000..d97279c2ec
--- /dev/null
+++ b/src/internal/utils/flatten-children.ts
@@ -0,0 +1,17 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+import React, { ReactNode } from 'react';
+import flattenChildrenLegacy from 'react-keyed-flatten-children';
+
+export function flattenChildren(children: ReactNode): ReactNode[] {
+  const majorVersion = parseInt(React.version.split('.')[0], 10);
+
+  if (majorVersion >= 19) {
+    // React 19+: Uses Children.toArray() which doesn't flatten fragments.
+    // This also supports bigint which is not available in earlier React versions.
+    return React.Children.toArray(children);
+  } else {
+    // React 16-18: Use react-keyed-flatten-children for backward compatibility
+    return flattenChildrenLegacy(children);
+  }
+}
diff --git a/src/space-between/internal.tsx b/src/space-between/internal.tsx
index f15e362fb2..8019045f74 100644
--- a/src/space-between/internal.tsx
+++ b/src/space-between/internal.tsx
@@ -1,13 +1,13 @@
 // Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 import React, { forwardRef } from 'react';
-import flattenChildren from 'react-keyed-flatten-children';
 import clsx from 'clsx';
 
 import { useMergeRefs } from '@cloudscape-design/component-toolkit/internal';
 
 import { getBaseProps } from '../internal/base-component';
 import { InternalBaseComponentProps } from '../internal/hooks/use-base-component';
+import { flattenChildren } from '../internal/utils/flatten-children';
 import WithNativeAttributes from '../internal/utils/with-native-attributes';
 import { SpaceBetweenProps } from './interfaces';
 
@@ -52,10 +52,11 @@ const InternalSpaceBetween = forwardRef(
         ref={mergedRef}
       >
         {flattenedChildren.map(child => {
-          const key = typeof child === 'object' ? child.key : undefined;
+          // If this react child is a primitive value, the key will be undefined
+          const key = (child as Record<'key', unknown>).key;
 
           return (
-            <div key={key} className={styles.child}>
+            <div key={key ? String(key) : undefined} className={styles.child}>
               {child}
             </div>
           );