A child with anal blood. What could it be is not constipated?

Fissure. Most commonly an anal fissure which are painful and bleed bright red. If persists, see pediatrician.
Too:big/fast/hard. If a kid is passing stool & it comes out too big, too fast or too hard it can trigger a crack in the skin that bleeds. It would be bright red & on the outside of the poop. If it is mixed well within a stool, it may be a sign of a blood producing pathogen like salmonella or shigella.