R.K.
I live in Virginia Beach and I have a 4 year old son. I sometimes find it difficult to find fun things to do. If your girls like nature at all, on Shore Dr there is large State Park called First Landing State Park. It is gorgeous, with many trails, camping areas, etc. Here is the website:http://www.dcr.virginia.gov/state_parks/fir.shtml
Also, there are the malls (Lynmnhaven has a kids play area and Merry-go-round). Also many bounce houses in the area.
The zoo is ok (I'm from San Diego, so I tend to compare this area to San Diego a lot). The beach is lovely, though windy and cold still. If you do go, Grommet Island is a great playground [you go South on Atlantic until you can't go any further, this spits you at Rudee Inlet where the park is].
You could also go to Portsmouth to the Children's Museum [I sometimes take my son to Waterside in Norfolk and take the Ferry over to Portsmouth, the first stop is pretty much accross the street from the museum].
All else fails, go walk the boardwalk :-)
Good luck and have fun!