my thoughts - don't declutter yet. help each of them pack clothes for the rest of this week (wed-friday) and a small bag or basket of anything else in their room they may need while they "camp out" elsewhere in the house, during the painting. pack up the rest of it into boxes or whatever you have on hand to get it out of their room. move the furniture that belongs in there to the center of the room and drape with plastic.
paint. set up the furniture the way you want it, with only the furniture you want in the room. make up the beds and do whatever else you plan to that will make it look awesome.
*now* it's time to declutter! now the girls see how cute their room is and exactly how much storage they have to work with. help them decide what goes back in to their room and what to do with the rest.
sounds like fun. do we get before & after pics?! eta: oops! of course we do! sorry, i don't know how i missed that in your op. i'm excited to see them.