I talked to the mom and she loved the idea.

We decided that the kids would go with us for Spring Break instead of waiting until June. With summer camps that all of the kids want to go to it could end up that our June schedule could change and trying to find a date that worked for all would be difficult.
I told her I was nervous about asking since I was asking her to pay for the tix and she said she was glad that I asked and felt we were comfortable enough with each other to be honest about if they could afford it or if we could afford it. She also said her and her dh would not have let us pay for it...they would have wanted to buy the tix.
She also said that her and her DH would also be giving us extra money on top of the ticket money because they know how expensive feeding teen/pre-teens could be. I told her that since it was a surprise trip our kids would be given a set amount of spending money since they won't have time to earn/save up spending money. She said we should get together before the trip and agree on an amount and she will send her two with the same amount of spending money.
We were going to do our trip reveal the morning we left...but now that the friends are going we are going to do a dinner with both families and do a big reveal. So now to plan that!
Thank you so much for all of the responses!!