A friend of mine mentioned this series of events called "OrKid Days" happening at Longwood Gardens. They involve kid-friendly music and traveling zoo animals. The next OrKid Days is tomorrow, February 20th from 12pm-3pm. Tomorrow, the Suzi Shelton Band will be playing and animals will be on display from the Brandywine Zoo.
If you're feeling a touch of cabin fever (and really, who isn't these days?), this may be something new and different to do with the kids. Plus, there's always the Indoor Children's Garden, open all year long. I have heard that it's pretty easy for the little ones to get wet in the fountains though, so you may want to bring a change of clothes.