This is a very interesting question and deserves and thoughtful answer. So here is mine. . . .I googled paintable crank bait blanks and got a bunch of different options. As far as colors go, option #A might be to use Testers modeling paint and brushes. Maybe get a fly tying vise to make it easier to paint all sides. Option #B might be to go to Michaels or Hobby Lobby or somewhere like that and get a cheap air brush. Maybe do both. Maybe just buy a bunch of different colored Sharpies - I don't know - I suck at arts & crafts stuff, which doesn't stop me from trying different things from time to time, with mixed results, but I know that I suck at colors & design and stuff and I don't have an eye for it.
I know people who don't suck at arts & crafts stuff and they tell me that decent gear makes a big difference, quality of brushes, quality of paints, how adjustable is the compressor on the air brush, whatever, JUST LIKE FISHING, DECENT TO GOOD TO GREAT GEAR MAKES A DIFFERENCE.
I'd be mindful that you can probably paint over the crank bait bodies more than once - or dip them in a white or bone color and start over.
I'd go to Michaels or Hobby Lobby and find out is there is anyone there that can tell you about modeling painting or painting on plastic in general. There are quite a few custom crank bait painters out there - call one or two of them, explain your issues and get their advice.
My advice (and this is advice from a guy who doesn't have children, doesn't want children, has many? probably more like some, friends and acquaintances, who have children, and grand children, and adopted children and grand children. Personally I don't mind children in small doses when they are well behaved & I have very little use for children who are whiney and ill-behaved ) would be to purchase a dozen or so blanks and lead off with Testors modeling paint & brushes - see how that goes. Get a fly tying vise - you wouldn't try to paint cranks without a fly tying vise and you shouldn't expect a child to. Maybe you might, I don't know. Then a few weeks after your first couple of attempts, get the child a decent entry level air brush & paints. It's only money and your child ain't going to be young forever. It is a good way to find out and encourage if your child has any art skills.
My younger sister was playing with my mom's air brush when she was around your daughters age. Of course, over time, she tore it up, misusing it and not understanding its limitations. Mom just bought another cheap air brush compressor and again went over the parameters of what to do and what not to do.
Hope this early am musing helps.