Here's the problem with the Baltimore Zoo: it isn't the National Zoo. With the National Zoo so close, so magnificent, and so, well, free, it's hard to remember what the Baltimore Zoo *is*--a really good zoo by any standards. Who cares if there aren't any pandas?
Over the past 15 years, the Baltimore Zoo has done a lot to renovate and make their exhibits more appealing and educational. They still suffer from their location (in Druid Hill Park in a part of the City that not many venture into regularly) but once you've navigated through those roads, the setting is beautiful and parking is free. That alone makes up for the difference in admission between the National Zoo and our own. Your kids will love it--the polar bear exhibit is always fun, and my daughter loves the prairie dog exhibit as well. Signage is very good--both for navigating through the Zoo and for learning more about what you are seeing. If you can swing it, go on a weekday in September--the summer crowds are gone and the school field trips haven't started up yet.