C.W.
Hi Mrs. C-
For my kiddos sharing a room when younger, I went with geometric PRIMARY colours for the sheets and bedding. I also got a nice primary colour kite with rainbow colour ribbon tail/streamers and suspended that from ceiling with streamers kind of billowing from the ceiling...
The kids loved it!
Primary was 'gender neutral'...and left wide open choices for other 'accent' pieces...
Best Luck!
Michele/cat