The ornamental design for a stencil for children, as shown and described.
The sole FIGURE is a top plan view of the stencil for children showing our new design. The bottom plan view is a mirror image of the top plan view. The front, rear, left and right side views are flat lines as the stencil has minimum thickness.
The broken line showing the outside edge of the sole FIGURE is included for the purpose of illustrating environment and forms no part of the claimed design.