From observation of my little guys (aged one and five), they like colourful pins that *do* stuff and where the objectives are simple.
My Flipper Beast (aged five) likes MM because he understands that you shoot the castle. He also likes Cirqus Voltaire because you shoot the evil ringmaster and his initial interest in our LoTR was driven by the Balrog. His interest in LoTR is now driven largely by the multiballs, which give a sense of excitement and progress to the gameplay. He’s a smart little guy and, after a couple of months, has some grasp of how to trigger them. Bigger screens are mainly of interest to the little boys because they make the game more interesting for ‘spectators’.
The Flipper Beast is definitely affected by themes, mostly castles and monsters. He likes JP2 because of the dinosaurs, GZ because of monsters destroying cities, MM because of the castle, and BK2000 because he is ‘battling a black knight’. Although he really enjoyed our FT when it arrived, he’s lost interest to some extent - possibly due to theme and also because it’s a very skill-driven game about achieving high scores.
Our Pi-Bo Wizard (aged one) likes gameplay videos of JP2 because of the T-Rex, but was really scared and bored by the T-Rex on the LCD screen in real life. He did, however, like GZ - probably due to a combination of the bright Zombie Yeti graphics and the colourful LCD screen. He’s also a huge fan of watching videos of TMNT and assume he would enjoy spectating the pin in real life - again, it seems to be down to the colourful playfield/backbox and lively graphics. He, also, turns up for multi-balls.
I know this isn’t representative of everyone, as they are both VERY little, but it gives a bit of a flavour...