My Daughter is very good friends with another girl in her class they are 9.
They have been very close from reception
the mum started taking my DD on play dates to hers aftershool .
She always offered I never expected and when I was off work I would always recip   
They were part of a bigger group of friends but this mum really focused in on my DD and her DD being friends but still we would meet up as a bigger group at the park etc on weekends.
Recently the mum has got quite off with me stopped offering my DD on playdates and when I was offering hers coming up with random excuses that she couldn't.
Now in school DD has told me the two of them still eat together and play together.
The mum has started taking another child on play dates which is completely fair enough but I don't really know how to explain this to my DD as there has been no fall out and tbh I am baffled by it too.
She has other friends that's not the issue I'm just not sure how to explain it to her.