What's the etiquette with parties?

[AnyDream]

We found with DD1 that the drop & runs started to increase around grade 2-3. Before that most parents would hang around, with a few exceptions (usually the single dads LOL). For a cinema birthday, I would offer to pay only for the invited guests (ie your child's friends) but I would ensure there were at least 2 supervising parents (eg myself & DH) because I would not want to leave kids watching the movie by themselves while I escorted others to go to the toilet (which is inevitable if they have drinks).

ETA - if the cinema visit is for a birthday (ie in place of a party) I would pay, provide drinks/snacks etc. if not, like if its just a movie date, then you'd expect the other parent to pay (unless it was a situation where you knew the other family was in financial hardship in which case you might say it was your treat). The other exception is when it's a family you are close to you might pay for their kids knowing that when your kids spend time with them, they pay - assumption of mutual reciprocity.