K.L.
I have not been there in years - actually since I was a young child (I am now 25). It is GREAT for children! We are planning to take my 2 1/2 year old niece there this summer. We live in Baltimore County and so it would only take about an hour, maybe 1 1/2 hours to get there so we will make it a day trip.
Another place to consider is Sesame Place. I remember going there as a child also and loving it there (and I wasn't even a sesame street fan). That is about 2 hours, I think.