If your kids are very good players, which it sounds like they are, there will be room for them on one of the area teams. I'd recommend that you start with Bethesda Soccer Club. Contact info for boys' TD is here:
http://www.bethesdasoccer.org/Default.aspx?tabid=225773 Our boys have been there for several years, and we think very highly of the program.
Other relatively close options are Potomac Soccer Club, and Stoddert Soccer in DC. If your older child is interested trying out for the Development Academy, both Bethesda and DC United have teams in his age group.
Since you are coming in the summer, I imagine that the clubs would suggest that you come out for one of their July or August camps or clinics for an evaluation; otherwise, you'd probably need to wait until practices start up at the end of August.