DecomposingComposers children who seek out and only interact with adult strangers and not other children do usually have issues of one type or another, whether special needs or neglect or indeed lack of social skills due to lack of exposure to other children without heavy adult direction you can't always tell. Yes children with that tendancy need parenting slightly differently. Most 4 or 5 year olds will not behave like that.
In fact I wonder whether this is a vicious circle sometimes. Where I live children of 5, certainly 6 plus have a fair amount of freedom and certainly go to the village playground alone. I've only encountered one child here in 12 years of taking my children to the playground and now just checking on the youngest, who sought out adult company rather than his peers, and he is autistic and struggles a bit with peer relationships. The other children are polite to adults but it wouldn't occur to or interest them to choose adult company over the other children, far less to play with an adult when there are children around.
Children who go to public playgrounds tend to play together. They also tend not to bring toys with them but rather to play with what's there already - it rather defeats the object of a playground if you take your own entertainment!
Mind you my kids live so close they do sometimes use the space if nobody else is there to play with their suction cup bow and arrow and target, but they know that if other children old enough to use it (7+) turn up they either take turns or pack it away and bring it home, and if toddlers turn up with parents they pack it up and bring it home.
I wonder what's creating all these 4/5 year olds who want to interact with unknown adults instead of other children? O we directed, over scheduled children used to looking to adults for everything and absolutely devoid of experience of managing without adult input, or neglected children? The number with special needs isn't going to be the main explanation, and parents of children with relevant special needs tend to be on the ball and know that their child needs extra support. Perhaps there are more parents at both extremes of suffocating / over scheduling / over directing children's time and the opposite, neglectful end, and too few in the middle, in some parts of the UK.
In the swimming pool example I'd be interested to hear how old the children were and whether they could swim. If they're over 8 and competent swimmers there's nothing wrong with them using a pool in a complex without adults. Their parents probably didn't expect anything at all of the other adults, and certainly not babysitting, drink buying or kidnapping them to take on an outing, nor escorting home - they probably just mentioned that there were other kids in the pool and maybe they'd like to play, with zero responsibility or expectations passed to other adults. If they were preschoolers or non swimmers it was obviously dangerous and neglectful... but with older children you get better outcomes if you let your children have some independence instead of keeping them permanently within arm's reach then suddenly chucking them in the deep end and making them completely responsible for themselves at some arbitrary age, or throw in the towel and give up parenting/ make parenting a battlefield when they rebel against the suffocating over supervision and it becomes impossible to force them to do everything with a parent.