I need help!!

Take some stuff from the little girls room like a small skirt, make it into a curtain, use the theme from her room into that, this way she will look at it as part of her room, also teaches her responsibility for it.

If it doubles for adult bathroom, I would use colors that compliment the girls room without being overly girly. If her room is done in pink or purple, I would choose a blush tone for bathroom, and perhaps have pink or purple towels for child and for adults, a grey tone towel with pink ribbon at edge. (if not used frequently by adults, I would place hand towels in a basket on a shelf with guest soap in wrapper with other small items for use of guest, ie: small hand cream dispener or tube). Shower curtain could be neutral with accents of the pink or purple. Nothing too dark as you said, bathroom was small. You could place a pictures using brighter colors in it to make it more homey.

I did somthing similar in my daughter's old room. It was painted a deeper blue (because it gets lots of sun, and this would allow guests to sleep a little longer) and I painted the connecting bathroom in a lighter blue. I had a mild theme of the sea, and added an embroidered sail boat shower curtain, two white framed pictures of the seashore, and on the counter, I have two small stand alone cabinets with a shell design to hold items like toothpaste, small, hand creams etc.