Google Fifth's disease (or slapped cheeks disease) and see if the images are similar to what your sons are experiencing. Easier than describing it all here - you can compare for yourself.
My children get eczema in the winter months. That can also just appear like rosy cheeks. However, they've had that since birth (the ones who have it). However, one of them - that's his telltale sign. No rash, no dry skin, not raised - just red skin on his face.
In that case, it's quite painful. So I doubt it's that. Just thought I'd mention it in case your kids have ever had it elsewhere.
Best to call :) good luck.