Along with the idea of bringing her back to the caregiver is to also convey the idea to the caregiver that you are there to play with your own kids and it isn't your job to include the other child no matter how much she wants to play too - that is the other caregiver's job.

If you do see the mother at a playgroup or whatever you could maybe bring up something like oh Susie looks so much cuter without her face covered in sand. I felt so bad for her when we were at the park last week, month, etc. she tried to climb the ladder and couldn't, got her foot caught in the ropes, fell into the sand. I tried to brush off as much as I could.....

then leave it at that as in actually try to get away at that point so you don't have to answer any more questions but you have put the seeds of doubt in the mom's mind. Then if she does confront you again about where was the caregiver you can say something well seh was chatting to another provider at the time but I noticed because I was in the sandpit area playing with my two charges.