I'll go ahead and vote DP. I think it's a game that's going to appeal to a wide range of player skill, has humor that will appeal to your son* and is pretty much universally liked. I think TMNT has reputation for being a little difficult and most Elwin games tend to appeal more to the serious player than the casual both in difficulty of shots and code. DP has a nice simple hook -- fight bad guys with a video game-like approach. Monster Bash is fun, but I think in a small collection it could get old faster than the rest.
Pinball is super subjective though, so take everything we say with a grain of salt.
*re-read your post and saw that your son is three, hmm . . .the humor might not be all that relevant yet