Near as I can understand it:

OP/OP's kid invited A to go to Universal over break. And A's parents are like OK we'll go along.

Then... A's parents decided to also bring B (sister) and C (another friend) on the trip.

So it's no longer OP's fam hosting a birthday trip for daughter and A. Now it's a group grope of OP's fam, A's parents, along with OP's kid, A, B and C. A,B, and C will be staying with A's parents so OP's kid isn't going to get the fun trip of pillow fight or whatever with A.

And the commenters are correctly pointing out that if OP's kid invites D as the alternative friend for the trip, then if they are going to hang out you've got 5 kids - OP's kid, A, B, C, and D (the alternate friend) - so five girls in a pack, which is always a recipe for a meltdown.

Hopefully I don't sound pedantic, it took typing it out to figure out it for myself.