At 6 and 8 children don't need to hold their mother's hand all the time - in fact they will be used to being away from their mother as they will be at school.
They had each other for company, and their mother wasn't far away.
And although it was one of the children who asked I wonder if this wasn't perhaps out of obedience (mother asked her to ask, so she did) rather than out of any genuine desire to have her mother sitting next to her. Most 6 to 8-year-olds actually ENJOY time away from their parents - it makes them feel grown up!
So - I don't think the OP's friend did anything wrong by not swapping seats.