In PhotoHunter, if the children cannot spot the differences, there should be a way to give them some hints.

An idea is to have a button that will display the two images centered one on top of the other and have a "progress bar" which will let children change the opacity of the top image so the differences will be easier to be spotted.
In this "help" mode, the child cannot mark any difference: he must leave the help mode first and then he will be able to tap on the differences he observed.