I am building my kids a fort/playhouse on an small sloped part of backyard. The 2 front posts will be at ground level, the 2 back posts will be maybe a foot above ground maybe less(these are on a slope going up). Does anyone know how deep these posts should be?

18". Would be more than adequate for a 1' exposure. Use 6X6 PT and you will have no trouble

I agree with ma, that includes the front posts being buried. Not knowing what your fort will look like or weigh you might need a 3rd row of posts mid way