Age limits on play equipment (bar because a child is actually big enough to break it) are often a grade A pain in the butt.
If you have two DCs and one parent, you often have very little choice, but to ignore them.
Clearly big sister will want to cool off too, or little sister follow her into the big ball pool.
What is required isn't age limits, it's good manners and common sense.
If your toddler is causing a huge queue on the big slide remove them. If your ten your old twins are having a massive splashy water fight amongst the five year olds, tell them to stop.
DD1 never stayed in the toddler area of any soft play, but she didn't care if the bigger kids sent her flying. She reappeared with a beautiful black eye once. No tears, she hadn't even noticed.
Likewise both of them are fish and totally happy to take their turn with the much older boys jumping off the pool bridge. They looked out for themselves and knew to swim clear politely and quickly.
The trouble starts when older or younger children take each other's space without respecting those who have most right to be there.
Teens, sitting on swings or playing football in the play area, when there is a pitch and benches, are my particular bug bear. Likewise if a tiny child took their scooter into the skate park and made everyone stand about that would be out of order.