Writing a card seems the kind of thing my 5 year would do. Just ask him to grab a coffee with you. Better not to involve the kids if you auctually want to date him
Probably just strike up a conversation with him and then casually drop it "haha yeah, it's a great show... hey, are you free tomorrow? Do you want to go get coffee?"
Personally I'd try and make it seem like it wasn't a date, just a couple of friends catching up and letting their kids play. But that's just the way I do things. Then I make a move when the moment feels right. Like after I get coffee I always have to walk around for a while, so I might maybe hold his hand while we walk or point out how beautiful the view is from here, and smile and make cute eyes at him.