With the amount of vitriol thrown around in American politics these days, I wouldn't want my kids directly exposed to a public forum like that if I was in the Presidential family.
Well there's also a significant danger of security leakage if the girls talk about what their family is doing, I guess.
I doubt Mr. Obama tells them national security related things, they're a little young for that. I have no doubt they would be subject to well organized and foul trolling if anyone could locate them online.
I don't think that's what he meant. Even simple things like "We're going to Disney World tomorrow" could be dangerous in the wrong hands, especially if the trip wasn't publicized beforehand.