I'm originally from that area so have a few ideas.
Rebounders in Timonium is a good gym. Lots of conditioning work which can sometimes be brutal to those not used to it, TOPs type stuff. But the coaches are great. I trained here at the end of my gymnastics career and wish I had been there the whole time. They currently have up to level 9 gymnasts. They also have an MDL team for more laid back competitive gymnastics with a really great coach. This is the only gym in the area I know a good amount of details about, but I know the names of a few others.
Baltimore County Gymnastics in White Marsh. About the only thing I know about this gym is that they landed at least one girl a college scholarship.
United Gymnastix in Reisterstown. I had a teammate who left this gym, but I don't really know the reasons why. She was the only one though, so I assume there are no serious problems.
There is also Towson University which has two camps in late June/early July that might be fun for your girls.
Towson Summer Sports Camps - Camp Details - powered by RegOnline There is the website for that.
Those are the only gyms I know of in the immediate Baltimore area, but there are some others as you move farther away from the city, so it depends on where you would be living.
Hope that helps a little! If you have any more questions I would be happy to try to answer them!