I’m in two minds having read your post saying that these visits to the park are when the older girls mum visits, presumably not every day. So the 10 year old is with a family friend for an hour or so and possibly enjoys being made a fuss of by the older girl.
When I was about the same age I used to love spending time with a teenage neighbour, she would take me to the park and buy me sweets and I had a great time, sometimes just the two of us, sometimes with her friends. They made me feel special and I think they enjoys “playing mum” for a short time. I was the oldest in a large family so had little time on my own, it was usually me looking after the baby!
I depends on the frequency of the visits, the length of time and if the two girls enjoy spending time together.