My girlfriend and her family are mad at me?

yesterday i was leaving from my girlfriend's house to go play basketball with 2 of my friends and my girlfriend's little brother wanted to come. hes 12. so i was like sure why not, it would be better with 4 people anyway. so were playing and once i accidentally plowed through the kid and knocked him over (i didnt mean to but hes small) he was fine tho only some minor scs. then later he was trying to steal to elbowed him in the face to make him back off and he started crying. me and my friends don't care about fouls so i always elbow them in the gut or push them over so i didn't think it would be a big deal. we had to stop the whole game and i went over to check if he was ok and saw that i busted his lip. so now i have a bawling kid with a busted lip and i have to take him home and explain to his sister (my girlfriend) what i did. great. as you can imagine i had a sh** day. my girlfriend and her family are pretty pissed at me but imean its not really my fault right? i mean yeah i did it on purpose but i didnt mean to hurt him (but that doesnt make sense to her)...