Matchstick
Adventurer
Might be able to get the guy 'on board' with an up-front role during play and by giving a little ground if he looks for a couple of rules interpretations/ checks. Which possibly adds up to - make sure he's enjoying the game and he'll probably not give a thought to whether you've applied the rules precisely?
Your kids's games are much more interesting/ important than a possibly grumpy player. I'm doubtful that starting on the D&D model and module mentioned is helpful if you'd like girls to go on and play D&D as they get older. Particularly when you're used to improvising.
Basically, are you looking for them to follow the standard Western movie narrative model with a bit of roleplaying or to build shared narratives with you (learning golddust)and learn how to choose how they play a system like 4e to a style of their and your own preference once they're ready for a few more rules?
p.s. whatever you decide please give them useful pets, let them train up the pets and never, NEVER kill one of the pets. They'll love 'em and you place yourself inside the narrative where it's less obvious that you're reviewing, reframing and prompting in-game. 'Of course the old wolf can talk, it's a fantasy game.'
We've started with the old D&D boardgame and I think they're ready to maybe try something a little more complicated. For the most part I think we'll be more shared narrative with me providing the reactions. The system we use will be pretty loose and take a dang far backseat to them having fun.
It's interesting that you mention pets. I was going to go with some pokemon (something they both like a lot) type pets that could accompany them. Maybe actual pokemon, but more likely some sort of more petlike equivalent with much more limited powers in a battle.
It'll be interesting to see if my wife will play too.
