What is your after-school schedule like? How much time for each activity (including homework, outside play, extracurricular stuff, dinner, getting ready for bed)? Anything else I'm missing? What time do your kids get out of school? Also, what time do your big kids go to bed?
My DS isn't in this age group but if I had it to do over again he would have a set study time. Even if there is no homework give him 30 mins to read or go over his hardest subject.