These are fine studies (or summaries), but they don’t say cellphones are good. They say that they can’t say whether cellphones are bad. For now, common sense is the most effective tool we have to deal with cellphones and kids. So I’d say, enjoy but do it responsibly. Much like drugs and alcohol. (In the Netherlands we have the public health slogan for alcohol “geniet maar drink met mate”, which is “enjoy but drink with moderation”. I think that sentiment applies here too)