I have a SIL who lives in Hyattsville but they don't have any little ones yet ... however there are a ton of families in the neighborhood. I've also heard of a listserv for Hyattsville moms but I don't know how to get on it, since I don't live there. Sorry I can't give any more specific information!
I live just down the road in Mount Rainier. We have one boy, turning 4 in September.
I'm new here, so I'm not sure how to suggest getting in touch -- I think I clicked the thing that lets other members send me e-mail. Please feel free to do so.
One thought: PG Pool is a great place to meet other families. Not cheap, but it's late enough in the summer that you can do a partial membership. http://pgpool.org/ And it's a lot more than just a pool. Drive by and you can see -- big lawn with picnic tables and grills. Huge kids pool, a sandbox under big shade trees. Sometimes they have bands play. Some nights it's open late and there's a campfire with smores. It's sort of a hippie country club.