Need some ideas for a bathroom walls - quick!!!!

We are re-doing the hallway bathroom of our 1976 ranch-style home. I find I have used a lot of pale, creamy yellow and sage green in the rest of the house. So these are colors I suppose I am partial to, although I'm definitely open to other suggestions.

This bathroom is the "powder room" for visitors as well as the main bathroom for a boy (age 13), and a girl (age 6). It's not terribly large, measuring about 5x8 (not including the tub and linen closet along one wall.) We've put down neutral taupe/gray tile on the floor, the vanity is a clear maple, and the countertop and sink are white as is the tub, toilet and (soon) the wood trim around doorways and baseboards.

Here's my goal: a bathroom that is neither too feminine nor too masculine; not too adult nor too juvenile. But at the same time, I don't want it to be bland - a tall order I know. Any ideas for wallpaper color/prints?

(P.S., the kids had a Looney Tunes cartoon border with squiggly line black and white striped wallpaper in their last bathroom, and if I can't come up with something quick they're probably going to try to guilt me into doing something similar in here.....this is a desperate, urgent SOS!!!!)