I too think it's worth it, as it's really no trouble to do, and you can later transfer RR points from one account to another (for a fee). I keep all the RR numbers in an email that I can search for and retrieve when making reservations.
One thing to keep in mind is that RR points do not expire so long as you earn RR points in any 24 month period, at least as I understand it -- and earnings RR points isn't hard to do if you're buying SWA tickets or renting cars/hotels with their partners -- but you don't earn RR points when you flying using RR points as we've been doing with the points accumulated on our Southwest CC. So if you're not buying tickets at least every 24 months using the kids' RR info, you may have to act later on to keep from losing accumulated points. I actually had to do that yesterday, by transferring some of my points into their RR accounts (for a minimum $20 fee each--but worth it to avoid losing points that are worth more).
But anyway, that's just a potential issue to deal with two years from now....I'd go ahead and sign them up now.