Just remember this is my advice and different councils do things differently.
Sorry you are having drama in your troop. Part of the reason I am not a fan of multi-level troops. In this case for now, it would not work after the grade level change. Had the 1st grader not joined I would have recommended that all the girls, have bridged to Juniors and then your troop would have been all Juniors.
In my experience the money should be split amongst the girls. Although you probably would want the girls to vote on this. So if you have 8 girls that break off into Brownies and 5 girls into Juniors, the money would be divided by 13. Take this amount and multiply it by 5 this amount goes to the Junior troop. Take the amount and multiply by 8 this goes to the Brownie troop.
I would be calling my service unit or neighborhood manager/director or I would call your membership representative at council. Just to give them a heads up of what is happening. I prefer to know what is going on ahead of time. Instead of later finding out that things totally blew up and now the girls want to quit.
to you as you navigate through this
Have fun at the Halloween party.
The thunder was shaking the house. 
