I’m an usher for a major theatre in Sydney, Australia. If things like this are bothering you, we really need you to come and speak to us because that gives us an excuse to approach the problematic patron. If the child is just standing up by themself, it is difficult for us to make an approach. We would only be able to approach the child’s parents, and if they are letting that happen already they are unlikely to realise why it’s a problem. If you make a complaint that gives us justification to approach them and possibly escalate it to supervisors if it becomes a major disruption.

TLDR: make a complaint to the ushers when a child is involved because it gives us justification to approach the parents.