re monetising. I assume you plan to bring out more things to colour? If so you could announce them being available when the app is first opened. Assuming that is most likely to be done by the adult it should avoid worrying people. Make anything which could lead to a purchase happening obvious and make the bright coloured attention grabbing button take the user (possibly the child) to the game.

unless there is a safer/better way to go about it. Good luck!