The most popular beaches are Ocean City, MD, Rehoboth Beach, DE and Bethany Beach, DE. All would be fine for your family.

Just for your information, earlier in history, people used to go to the Chesapeake Bay beaches, however, the Bay is full of stinging jellyfish during July and August, and there are not many facilities for tourists there. Since the 5-mile Bay Bridge was built in 1952, everyone goes to the Atlantic Ocean beaches instead (like the ones named above) where it's better. Try to avoid crossing the Bay Bridge eastbound Fridays or westbound Sundays, as the traffic is heaviest then.

We always go the the Jersey shore. It takes about the same amount of time to drive from Baltimore to Ocean City, NJ, as it does to Ocean City, MD. Both Ocean City, NJ, and Wildwood, NJ, have boardwalks with amusement rides. You might enjoy a day trip on the ferry from Cape May, NJ, to Delaware and back.

Ocean City, MD is always fun.

Another place our family loves to go is Chincoteague, VA. We go every summer.

As mentioned, Ocean City, Rehoboth, and Bethany would be good choices with a family. Of those, OC will have the nicer/larger hotels and probably more activities for the kids. Rehoboth's boardwalk would keep them entertained as well, but your hotel choices in OC are better. From what I've seen, Bethany doesn't have much more than the beach and a couple of restaurants, but I must say that I've only ridden by it when going to OC for the day from Dewey Beach (which you do not want to take a family to).

Traffic on 50 out of DC will be a zoo after around noon on Friday's and just as bad, if not worse, between 8:30 a.m. and 2 p.m. on Saturday. If you can work it in your schedule, maybe try to hit the beach during the week -- cheaper hotel rates and less traffic getting out of the DC area.