If the child was doing something dangerous or spiteful that was affecting your child? Parents nowhere to be seen and not intervening.
Have you ever been in one of those situations out and about where another child is doing something spiteful/annoying such repeatedly pushing in a queue, bothering your child or taking their stuff, or even doing something dangerous.
Like for example I was at the park and a small child was going round with a big tree branch whacking all of the play equipment while it was being used. Or another time my child was taking their turn on something and another child tried to physically take it off them.
Do you speak up and ask the child not to do it?
Yanbu - you would speak up
Yabu - say nothing