I've been wondering this too. I have 2 boys, ages 2 and 4. I'm planning on one outfit for each day (so 7), plus 5 extras. I think we will take a break that many days, so that allows them to change into clean, non-sweaty/dirty clothes after the break. If we don't need them, then at least their clothes are so small we're not out too much space. I'm putting each outfit into a gallon ziploc. When you do that, and squeeze the air out, they really get small!
As far as diapers, how many do you use each day now at home? My son uses about 5, so I am planning that many times 7 days, plus 10-12 extra just in case. If I get desperate we can get more there, but I think we'll be fine.