The lower levels are easy enough for 4/5 year old but as you progress, parents will have to help with the logic and reasoning of choosing moves. Still, it is a great app that will have your child thinking about the best course of action. This app works for solitary play if your child is a bit older.
My almost-five son and I play this game together, and talk about how to solve each of the puzzles. While some of them are difficult even for me, he often comes up with good ideas for solving, and we'll try our ideas out until we get it right.