I'm really not sure about this to be honest. The police officer in me categorically agrees that there are no kids in GTA, I mean, Child (Sexual) Exploitation is a massive problem, and putting kids in GTA only to get shot, blown up or run over by some mentalist would not please a lot of people. However, the gamer in me thinks it would add immersion, and, as you say, new callouts for LCPDFR.
I don't think it will ever happen, if they were implemented I guarantee it would be banned in a majority of countries the game is published in.