This depends... do you actually want them to be roleplayers as adults?
In that case, I would strongly advise you NOT to let them roleplay now. Let them play with action figures, maybe even miniature games when they're a little older (like heroquest), but no matter what you don't let them play RPGs with you "Until they're 16". Make a point of saying that.
Odds are that just to defy you, around the age of 14, they'll start running their own campaigns.
Then make a big deal, when they turn 16, of inviting them to your regular gaming group.
Its a plan for the future, but I can guarantee you that its far more likely to see you roleplaying with your kids in your old age. If you start roleplaying with them now, the second they're 13 they'll see it as the "lame thing dad does" and be totally uninterested for the rest of their lives. I've seen it happen.
Whereas if you make it a forbidden fruit and a trial of manhood, they'll be hooked for life when the time comes. I've seen it work, and made it work in my own case with my younger brother (who's a decade younger than me, and now also a lifelong roleplayer).
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