Interesting how the possibility of the daughter exerting her boundaries as an adult is being framed as "being a huge baby."

The good news is, you've made it clear to your daughter that she has three choices:

1. Do what you say because of the golden rule ("he who has the gold makes the rules").
2. Pay for her own accommodations and remind anyone who critiques her for sharing a room with her partner that hey, she paid for the room, she can do what she wants.
3. Opt out of the trip altogether.

You've done her a huge favor by demonstrating that a strong no and financial independence are much more pleasant than dancing to someone else's tune. Enjoy your trip.