Putting the moral question aside for a minute, on a purely practical level lots of people ignore / don't read signs.
You only have to look at age segregated play areas (and IME it's parents of very young children who are the worst offenders - little Jude is such a good climber and so fearless, so he uses the over 8s play area instead of the adjacent younger children's play equipment even though he is only 2 and the older children must all moderate their play to be careful of him).
No dogs off leads in the park signs, no dogs on the beach full stop signs, no climbing on the ancient monuments, no feeding the ducks bread - all get ignored.
Given that people generally don't read signs in all areas of life it seems lip service at best for a council to say they are "putting on" a session for one group in society if "putting on" the session just means social media posts, an ad in the local paper and a sign.
An employee of whoever was running the session making loud, clear announcements every 15 minutes from 4 pm onwards that from 5pm the splash park is for the exclusive use of special needs families might have done a better job of clearing out those who that did not apply to who were blundering on in their own bubble not really paying attention to signage ...
It would seem wrong in an unfenced area (nobody seems able to say quote how splash parks are set up, whether they are fenced so that who enters can be monitored or just part of the park) to challenge individuals about whether any of their children had special needs on the spot, that doesn't prevent regular announcements and somebody official looking in a council employee/ whatever company or organisation puts on the session T shirt making announcements which would get through to people who shouldn't be there better than an easily ignored/ missed/ misinterpreted sign.