Howard University has very generous guaranteed merit scholarships, and your son would qualify for the Capstone Scholarship which includes tuition, fees, and room for applicants with SAT CR+M 1300-1390 and GPA > 3.25. I know Washington D.C. is a little below Philadelphia geographically, but maybe the generous scholarship would make it more appealing 
Plus if your son is interested in International Relations, there is no place better to be than Washington D.C. If he raises his SAT score by just 20 points (!) he would qualify for the Founders Scholarship which is tuition, fees, room, board, and a $500 book voucher. If your daughter can raise her SAT score by 200 points, she would qualify for the Legacy Scholarship, which covers tuition and fees. If she would enjoy a state school, I think she would like Howard (despite it being private) because they definitely have a lot of parties.
Temple University is also great for merit scholarships, and your son would qualify for the $18k scholarship with the $4k summer stipend.
If your daughter can raise her SAT score by 150 points, she would qualify for a 35-40% tuition scholarship at University of Alabama at Huntsville, which has a COA of $32k.