What should we do if we believed someone is in danger or at risk in some way?

Well , it is hard to just grab the parents and point your fingers to their face but it is useful to say something that their kids are going to fall.

In my part I do not want to see a kid bleeds his head because I just keep quiet.

If I really think that something bad will happen and the parents are not aware of the situation, then I will let them know the danger ahead.

I think yelling or shouting is not the good idea to start with but I guess you can do it in a manner that they won't feel stupid.

You cannot just scream to them and then everybody will get notice of what you are doing.

You can tell them and approach them politely in a way that won't hurt their feelings.

If you help someone but then you act like you are the boss and then raise your voice , they might just feel embarrass.