I'm sorry this is happening and I think you should basically perform a coup. This lady should not be allowed around any children, much less under the guise of a GS leader.
I would call the leader and tell her how things are going to be. Don't play dumb and don't ask for explanations as this gives her a feeling of being in control of the situation. If she tries to interrupt, tell her that you are not finished and continue. You are in control. First, tell her that you were informed that your daughter was not invited to a troop function and that she, an "adult", was behind this. Tell her that this is not only unacceptable behavior from an adult to a child, but that it is also ILLEGAL. Tell her that you are not only reporting her to the council, asking for her removal from your troop, but also asking that they ban her from any future GS work. Tell her that if she does not IMMEDIATELY resign from her position then you will do 2 things: 1.) hire a lawyer and 2.) go to the press. The news would eat this up if they had it, and I'm sure she'd love if all of her rich friends found out she was picking on a child. I would then wait for her response. If she goes on and on, yelling even, calmly ask her if you can expect her resignation by the end of the day. I would also somehow work in that your daughter WILL be attending the Six Flags event, that you will be going as a chaperone, and that her daughter is welcome to attend as well, but you would prefer if the leader had no contact whatsoever with your or her daughter as you would prefer your daughter not to be associated with people like her.
Keep it short, be firm, and knock her socks off. She won't know what hit her.