I am the proud parent of a 5 year old, 8 year old, and 10 year old. Kids just don't care about going downstairs and playing pinball the way that we do. However, that does not mean kids can't have fun playing pinball. If I play pinball with them and actively encourage them, they will absolutely have a blast playing. They could care less about the rules, as I'm usually actively coaching and joshing them when we play. I have noticed the same thing on my MAME and Arcade 1ups. They don't play on their own but if there is a social aspect to it they will have a blast not matter the game. They will ignore Outrun, but will happily go through mindless beat em' ups and shoot em up games so long as I am with them and we are having fun together. I can set them up to play together and they will have a blast. When the kids have friends over they always come downstairs and play a while, but a good arcade session is maybe 20 minutes for them before moving on to legos (usually).
Out of everything I have put into my arcade, honestly... the used ping pong table I got off Craigslist has been the greatest bang for the buck. I am eyeing a pool table (with ping pong cover top) as a replacement soon or a bumper pool table with gaming top.
I will say on choice of games, kids tend to me more drawn to mechanical gadgetry than they are to deep rules and "good" games. In my experience with my kids the biggest hits have been World Cup Soccer (great toys, a puppy on the backboard, and goals to shoot for), Data East Jurassic Park (the dinosaur eats the ball!), and Medieval Madness (look at what the castle does daddy!).
Kids could care less about themes and rules.
As an afterthought, I am guessing pinball is the closest many of us here get to traditional video games these days (not everyone here). Pinball is attractive to kids for the aspects that are distant from video games from their perspective. Those items are the mechanical nature and toys, and the social aspect.
Make it social and kids love it!
Edit to ad: In other words, kids won't care about or even really remember playing pinball at uncle Jimmy's house... but if you play with them, encourage them, and make it a social activity they will sure remember playing pinball WITH uncle Jimmy.