Via MyPuzzle.org I found a nice online version of Sudoku for kids.
The Sudoku grid is 4×4 and has four colored squares of 2×2. For this Sudoku you’ll only have to use the digits 1 to 4. Each row, columns and colored square must contain one of each digit.
This kids version of Sudoku has three levels (easy, standard and difficult) and hasn’t (as far I found out) an unique solution for each puzzle as in original Sudoku’s.