For the guidelines you followed, it was very good. I know it is meant for kids, but I still found a few things that I think could be fixed up. On the find the difference puzzle, there is an (I assume) unintentional difference involving the level 1 door blocking an object. Also, entering the proper code before the 'invalid' one given to you results in access denied which I don't think should happen, but not a big deal the first time you play through. Lastly, I agree with a few others in that there should be an ending. Perhaps something in the safe that would possibly be a big clue to the detective as to who the killer was.
Aside from those minor details, it was good for a kids game and stuck well to the guidelines of the project you were given. Good job.