another idea I have seen around the traps is a cake (whatever flavour/ however many layers etc..) covered in white fondant... then you get yourself a packet of edible food colour pens (google it if you dont know, FooDoodler, americolour etc) then let the kids doodle all over it! draw, write etc, whatever they want!

you could also do two smaller cakes, so they have one each?
I love this idea, will be doing it for DS in a few years