I took a 12 year olds dose of pain stop once when i had no other option and it knocked me out cold for 3 hours! (When I was 24) Thank god a good friend was with me and she hung out and looked after DD with her kids while I slept.
Having said that, i gave it to my DD when she was 5 and had chicken pox so badly it hurt her to lay down and nothing else was working. She felt better but hardly even got drowsy!
Different for everyone, stick to the reccommended dose and hope she feels better soon!