I can only help with the what to do in Frederick MD part. Just north of Frederick is a small zoo (nice shade) called the Catoctin Mountain Zoo. My kids loved it when they were small (and preferred it to the National Zoo which is built on a hill and is very hot in the summer). DC is a good option from Frederick as is Baltimore. Baltimore has several cool kid things including the Aquarium (pricey but well worth it) and Port Discovery (which is geared toward younger kids and could take almost a whole day). There are also harbor boat rides that the kids might like and Ft. McHenry. If you do DC, you'll need to be prepared for traffic on 270 at any time of day. Best bet would be to drive to Shady Grove Metro Stop in Gaithersburg and take the Metro into the city. I would definitely suggest you bring your own stroller(s) with you. Some places will have them to rent but others won't. The DC Metro system is stroller friendly. Good luck on your trip!