I signed our kids up years ago and enrolled them in the Aeroplan kids thing. I think you can do it after the fact. My 11 yr old got a card this year for his birthday.

Look at Lufthansa though, to see if it makes sense for your kids. Once our kids hit 25,000 miles in their AP accounts (enough for a NA award) I switched them over to Lufthansa Miles & More. Last year, flying on the same flights as my wife they got approx 27,000 miles compared to the 19,000 she got. We had a couple of C/J fares in there that helped (LH has a better COS bonus), but Lufthansa routinely gave more points for discounted domestic fares. Since they don't have credit cards or long distance plans, all their miles come from flying.

I've tried to convice the kids that they have to speak with a German accent when they fly now, but they're not buying it.