Evening (or is it morning?!!) Jessie
and 
for you, sounds like you need both
I posted yesterday that the whole situation sounded tricky as I could see all sides of it and Spero, a poster who I admire btw, pointed out that yes, whilst it is tricky it musn't mean you shouldn't call.
Here's the thing. I don't think I am a great parent. I am doing my best.
I have a couple of golden rules in life which are:
Don't ask questions you don't want to know the answer to and
Don't shit where you eat
I was thinking why I would have been loathe to ring SS and thinking about it, I guess because I would hate if someone did that to me, because my kids are always shrieking for example. I would hate the involvement of outside agencies and the accusations that I am crap.
And so I don't like the idea of doing that to someone else who, like me, may well be doing their best.
But bottom line - I might not be the best mum in the world but nor have I ever left a five year old alone on a park. Nor would you. Nor will we.
So please be gentle with yourself. Please don't feel bad.
I have been in situations where I have called for police/ambulance. Once when there was a man lying on the road with blood on his head at night, I stayed with him then. Also I rang and reported a group of kids aggressively chasing a kid which I saw from a bus once, I will probably get flamed for not getting off the bus and stopping this myself now. I have also phoned police in domestic violence situations.
You should not need to defend yourself like you do here as the above situations did prompt immediate action and you swung into action. A child on her own in broad daylight on the swings may be completely benign and had she been older, not necessarily a cause for concern (which incidentally is a misnomer as teenagers can and are equally vulnerable).
I had a rough adolescence: not as bad as some but not golden either.
Have taught teens who liked detentions as it meant they could be with me rather than going home 
: alarm bells. Luckily I was able to pass many of those cases upwards to Child Protection colleagues.
At six I was a latch-key kid with a slightly older sibling - daresay many here would have wanted SS involved then too. Perhaps that is also why this thread has resounded with me somewhat.
So fwiw I think ringing SS on account of a child being alone on the swings is a kneejerk reaction and as noted by others - is not an immediate reaction when a group of kids are playing alone on the street for example. But
what makes this different is by your account your own intuitions sense all is not well. Plus no other neighbours' kids to your knowledge are playing alongside this girl.
You posted perhaps as you felt guilty for her falling off her bike in her haste to catch up with you. This did not make you responsible for her accident. But you should have checked she was okay rather than assuming it - this you know. If there ever is a similar situation you can check before explaining he/she does not go off with strangers and ascertain what their parent has said. That can then lead you onto ringing SS if the child's reply infers the parent has effectively abandoned her.
I, for one, do not see you as a terrible person. You have now rung Crimestoppers, that is a good thing. Sod whether they thought you were stirring, you did something.
I don't think you need to move house. It is unlikely that if Crimestoppers do drop by the playground or take any action at all that your name will come into it.
See my second rule and the fact that I am a coward - nonetheless if I were to see the little girl is frequently at the playground unattended and by the fact you have already seen her twice there alone, it could well be a daily occurence in the holidays then you do not need to feel guilty picking up the phone to SS. Do it anonymously. You don't need to leave your name nor do you have to move house because of recriminations from neighbours.
Nor should you feel guilty.
Because you wouldn't leave your son there. And nor would I.