There wouldn't be much point to a sn session where everyone could go, surely, that is just what the park is the rest of the time (ie everyone can go!)
I have seen baby & toddler sessions at the water play thing in the park where we used to live, there were no staff to monitor, it was just a sign up by the kiosk, and if you turned up with an older child, someone would say, 'oh sorry this a babies only session'. It seemed to work ok!
Tbh ds would be beside himself at the change in plans and beside himself if it was overly busy, but it would be worse if it was overcrowded so we would have to go with the going somewhere else meltdown!