You could probably make quite a nice scheme with red and white - for example red with white polka dot wallpaper or curtain fabric. If not polka dot maybe gingham. Maybe get a similar coloured eiderdown to sit at the end of their beds to girly it up a bit.
Red and white sounds like a lovely idea. Would the groovy chick stuff you've already got fit in?
Or could you go with your original theme and let there be a bit of a clash just while the man u phase lasts? You could get 2 sets of duvet covers so that at least it'll match when the man u set is in the wash!
you could do what ive done with my living room..ish. walls are pretty neutral with natural wood furniture, fireplace etc but i change my sofa throws, curtains and ornaments according to season/how bored i am. red in winter, blue in summer, green if im bored! you could paint the walls white maybe and (if you want to spend the cash of course) buy 2 pairs of curtains, one for each colour scheme...perhaps if you wanted some colour on the walls you could get something to hang on them? one red/one whichever other colour you choose. if you have a wooden type floor that will go with anything too. when you wash the duvets, just change the other things round to match? if you still want it girly could you add some voile somewhere? or girly "textures" in the appropriate colours, cushions and stuff?
Red and white always looks quite sophisticated to me, so I'd go with that. I'ts not very girly, but it's not footie mad either. Alternatively, they can have their Man Utd covers, just shove a lovely pink throw over them when they get up, to blend in with a pink girly room.