How much time served for injury to a child (bodily injury)?

A 21 year old hit a 13 year old
What happened was the 21 year old wanted somthing to eat from the fridge (they are family and live together so) as the 21 is leaving the door with the snack the 13 year old threatens "give it back or I'll hurt you" and when the 21 year old turns back the 13 year old has a dirty look and a bundle of pencils in his hand the 21 confronts the 13 year old asking him "just you try, acting all big and bad" then he punches the 13 year old I'm the chin. The kid cries and tells the mother the mother is a criminal herself for her own problems and has been trying to get rid of the 21 year old (she doesn't care about her kids until someone is watching plus not that long ago she had attacked the 21 year old for no reason) anyway she calls the cops he goes in and serves for a past warrent and is in for 5 hours comes back and yesterday they had arrested him. The kid had a red mark at school yes. But it faded away the mom states he had a concussion but the boy had gone outside regardless and sat on his iPad completely fine (concussions causes pain when they look at light or loud noises etc) the kid did not tell the cops he had started the argument nor threatened. I witnessed this and this kid does not understand what jail is like for someone he is immature and this irritates me. The circumstances were stupid. This is Texas
Added (1). He was arrested yesterday for the assault to child.
This is his first domestic violence or assault to child.
Added (2). He's living there for the time being cause the only choices he's got is his brother (lots of trouble) so it's very difficult once he's out of course he's moving out he has had enough of his mother

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