PGT/Park girl (8)/Older/Yes/Go up and ask why she's here alone
You plan out a way to approach her. Showing concern for why she's here alone sounds like a good foot to start off on. After all, a kid her age should be suspicious if an adult does NOT approach from a position like that. Adults make things difficult for kids, always asking them to follow the rules. Showing that you are the same should make her drop her guard a little.
"Hey there, I'm sorry but... can I ask you something? It's not good for you to be here by yourself, where are your parents?"
"Ah... umm..." The girl responds, looking a little flustered. She ultimately does not answer, but she also does not seem to look all that suspicious of you. Seems this was the right approach.
You heave a big sigh and approach closer to her, climbing up onto the jungle-gym like climbing structure with her. "Look, it's dangerous for a little kid like you to be by yourself at the park. You need an adult with you." You breath another sigh, you are taking a horrible risk of that spooking her off, but if this works then it should blow past a lot of her defenses. "Alright, I guess I won't bother you about it too much, I can probably see if somebody comes and tries to kidnap you or something. Well, can you tell me your name?"
She looks down at the ground through the bars of the climbing structure as you climb up to the same level as her. "Umm... Amilia." She says.
"Amilia? My name's Larry. How old are you Amilia?"
"Umm... I'm eight."
"Yeah? So then, why are you here at the park alone?"
"Mmm..." She went silent again.