Was it wrong of me doing this ?

We had this family gathering and my son (4) was playing with his cousins, one of his cousins (4.5) keeps hitting and taking the toys from my son. And my son is not good in protecting himself (i dont know why but he is SO passive) and he recently became a big brother so he is having big feelings and cries alot.

Anyways , long story short after another fight i told his cousin that if he hits him again Im gonna tell his uncle and he got offended and went to tell on me to his parents. And i tried explaining the thing but they were offended.

I didnt mean to hurt anyone but their son was just so much , and they didnt stop him.