In the case of the play centre I would (and have) reminded older children that this play area is for children under 5. Then point them to the older play section. With the parent so close it makes things harder as they know the rules so they obviously don't care they are breaking them. Tricky.
I have been a total cow with this, and said stuff like "It's a shame that some people choose not to follow the rules, isn't it?" to the kids I'm with, or, once, in the case of a woman and child feeding ducks half a metre from a "don't feed the ducks" sign, a four year old asked me why. I said "Well, it seems they're not clever enough to be able to read. Why don't we go and read through the sign?"