If C and A were creating a group of kids to join an activity, and at the last moment a day before the session was to start C realized the time of the activity needed to be later, and A refused to come that late so C agreed to split up (orally on the phone), and find replacements for the kids C took out, and fill in the remainder of C’s time slots.
After 2 hours C found replacements for the 2 groups at their respected times, and called A, and A decided they did not want to cancel anymore so C should tell B that their slots are no longer available because A had been an initial one to form the group together with C, and A should have first rights over B even if A did cancel a few hours earlier. B cannot go to the earlier group as her kids are still in school at that time (same problem with C), and the size of the group cannot be greater than the amount signed up.
What should be done?
I am assuming that there are no written agreements here, and everything was done by oral agreement. You made up verbally and you canceled verbally. It is true that A originally formed the group with C, but that group was already split into two groups, and it is no longer C’s group anymore. Therefore it would appear that A does not have a right over B.
Aside from this, it is important for all parties to try to make it work, as shalom is very important… most probably more important than the group activity itself.