M.M.
Have them do a Neti Pot (get it at Walmart/Target etc) then if they are over 4, give them children's Mucinex. I can't take sinus medication except Clariton (it is the only one that does not wig me out) but I use the generic version. Doing these three things will GREATLY reduce the phlegm! Also, this is the time to teach them how to hawk a loogi :) Hope they feel better soon!