Not every playground has separate equipment for different ages of children but the playground the OP was commenting on did. Those "recommendations" are there for a reason - to keep the big kids off of the little kids equipment and to keep the little kids off of the big kids equipment. If you let your toddler play on equipment that is posted "for ages 5-12", then it is your responsibility if they get knocked over, pushed down or hurt.
ITA! What would anyone think of a few older grade schoolers 'invading' the toddler area of the park? The OP asked about a park where there were TWO playgrounds and it was posted. So, these other situations of only one play set, not posted, etc don't really pertain.
I am the mom of a 'split family'. I have four boys ages 13, 9,8 and 18 months. My 18 month old has NO business slowing down big kids on the posted 'big kids' playground. It's posted for kids above a certain height (I forget what it is). It is what the area is designated for as far as usage goes. My older kids don't belong tearing through the toddler part of the playground either. Once again, that area is designated for the use of children under a certain height. If the park is not busy, I will take my younger son over to the big kids equipment and let his brothers help him down the slide or take him across the monkey bars myself (holding his body and letting him think he is doing it). If I notice that others arrive, I will take my son and go back to the other part of the playground, the one that is set aside for his use. My older children are allowed to play WITH my younger son at the toddler playground with a strict reminder to watch for other small kids. If that area gets crowded and we all want to play together, we just head for another area of the park. I don't think my family's wants should come above anyone else's.
Think of it like this, when you see a basketball court at the park, do you think it's okay for toddlers to use that area for rolling around balls and that anyone who wants to use it for it's designated purpose should have to wait or try to play around them? I have seen many older kids and adults playing near each other without injury but when you throw little ones who don't have full control of their bodies (and with rare exception, this IS the case for most young toddlers), who don't understand dangers as well and/or aren't tall enough to be easily seen by most people enjoying the park areas for their designated usage, I think you are asking for someone to get hurt.
Even discount age...is it okay to picnic in the middle of marked soccer fields and then expect those who show up to play soccer to play around the picnic or wait until the picnic is over? Should the people who are there to play horseshoes have to be careful of the toddlers who want to make mud pies in the horseshoe pits or even other adults who want to use the area for yoga or something? Of course not! While it's okay (in most cases) to use the park in a variety of ways, if an area is designated for a certain age group, activity or purpose once other people show up those who are there to use the space the way it was designated should get to do so.
Parks have long had certain areas set aside for particular uses. While I might take my kids to play whiffle ball on a soccer field, I would (with no questions asked) yield the field back to anyone who came to use it for it's intended purpose. That said, if I had my toddler over on the field kicking a ball into the nets, I wouldn't hop off just cause bigger kids show up. I would however be mindful that it is a public park and not our private backyard and I would yield the field to them after a time and if it became apparent that they wanted to play a full field game.
Just my two cents.
