We stayed off property over the summer, so the room decorating was all on me. It was the first trip for the girls. I woke up super early on park day and decorated, and had a letter from Mickey telling them that they were going to WDW that day. It was wonderful, can cost me maybe $50 (the decor included autograph books, ear hats, sunglasses, and first trip t-shirts among other things).
I can't imagine spending $400 for them to decorate a room, that's a couple extra park days for a family of four, or a couple character meals.
I bet with a little creativity and $50 you could make it look better than they could anyway. What if your husband and the kids head straight to the pool while you "check in" and go set up the room. Afterwards, throw on your suit and go swim with the family. It will be a total surprise when everyone goes to the room together and sees the decorations.