Is it ok to discipline a total strangers child?

We were enjoying our day with family and friends, feeding the local ducks and riding bikes at the park.
Later at the Splash pad, a little girl in stripes came up and was clearly dumping a big gulp size cup full of water on my 3 year olds head. Watching and waiting for his reaction I was surprised he just brushed it off and went on trying to play. After the 3rd time of dumping the water over his head our friend’s daughter, T moved closer to scope out the situation. She stuck up for him, telling her to please stop. As she was mid-sentence the little girl in stripes took that full cup and hit both of T’s legs. T just looked up at me, as I started walking fast to get there. The little girl in stripes did not notice me coming up from behind to protect the kids. All while walking up there I searched for her mother. Nowhere to be seen. To me, a 5-6 year old girl shouldn’t be left alone especially if she is a bully. I walked up next her and touched her arm as not to frighten her, she looked at me as I was blurting out “that is not nice, there is no need to hit these kids.” She looked at me and told me she didn’t do it as all the others were shouting what she had done. I told her I watched it myself and that she was not allowed to do that again! She ran about 10 feet turned and stuck her tough out at me and kept running till I couldn’t see her anymore.
This left me questioning, is it ok to discipline total strangers kids? What if you don’t see their parents or the person responsible anywhere?
What would you do?