Because HOLY SHIT! I remember growing up there was SO MUCH controversy surrounding the GTA franchise based off of the simple fact that you can:

Bang a hooker
Rob said hooker
RUN OVER said hooker after robbing her.
Can you imagine the storm that would engulf GTA 5 if you could run over kids? I’m sure Rockstar has considered it, then went to Youtube and went “Uhhhhhhhh…. maaaaaaaybe we shouldn’t give them THAT much freedom…”

Pretty much thats my thought behind why there are no kids in GTA 5.