My daughter is 7. She loves AC/DC, mainly for the light show and ringing the bell. Or more specifically, making the bell hit the glass... She also loves WHO dunnit. Those are her top 2 favorites, but she kicked up quite a fuss when we let RBION leave the house.
SO, AC/DC, W?D, and RBION. Probably not the ones people would peg as kid-attracting themes. It's so easy to underestimate what kids are capable of and where their interests lie these days. They like the Wizard of Oz movie, yes, so that won't be a bad thing. But the focus has to be on one thing, and one thing only: fun.
The difficulty of a pin and the general ball time is a more legitimate area to focus on to try and attract new players. If a new player can make SOMETHING compelling happen in the first session playing a pin, that, above any assumed demographic-targeted theme, will have an impact.
Avengers is a majorly hard pin for me, but my 6 year old son likes it. And is probably as good or better than I am at it, if the frequency of multiball he gets is any indication. But that's what I'm talking about. He knows it's possible to get to this super cool multiball and Hulk bridge raising experience, and that is a great incentive to keep playing, get into it, gain experience, and maybe eventually start to understand the overall strategy.
That's a tricky divide. Seasoned pinball enthusiasts want a deeper more challenging experience. Newbies want to make something flash and go crazy in the first 5 minutes. That seems like more of a barrier to me than the theme.