I read this as another person who knows the parent in the last panel well letting the kids go wild playing make-believe and playing along. Then the parent in the last panel is teasing them for getting so into it themself during the three days the kids were with them.

There wasn’t even alone (as in “I left the kids alone with you…”) that would imply that they normally take care of the kids together.

My first thought was grandparent or auntie/uncle.