Kids can choose to solve from 4×4, 6×6 and 9×9 sudoku puzzles enlisted in this post. In a 4×4 sudoku, kids are expected to fill each 2×2 box with a number between 1 and 4 so that no number is repeated in each column or row. Similarly, they can fill the 6×6 and 9×9 grids with numbers 1-6 and 1-9 respectively.
You need to click on an image to get redirected to its PDF version so that you can take a printout. Since the printables are designed for kids, you will find that most of them are pretty easy to solve.