Here is a helpful solution. Number all of your puzzles, then write corresponding numbers on the backs of all the puzzle pieces. In case you find a missing piece, you will know exactly where it goes.
Completing a puzzle can sometimes be a challenge for little ones. Give them a helping hand by creating an outline of the puzzle. Remove one puzzle piece at a time and draw around it with a permanent marker. Knowing the shape of the pieces can be a helpful tool in helping children complete it.