I think OP is talking about playgrounds that have water features in the summer (for free). The playground situation in the outer suburbs is pretty pitiful compared to closer-in areas. There aren't any free splash parks out there--only pay parks like Signal Bay (great for toddlers, but not free) and Bull Run. I think it is expensive to run the splash pads b/c of the water cost (but that doesn't really explain why there are so few nice playgrounds in general out in manassas and prince william county--especially when there are so many kids).
The only (free) splash playgrounds I know of are the 3 in Arlington, the ones in DC (where the municipal pools are also free) and the ones in Montgomery County. Oh, and the one at Fair Lakes, but that isn't really a playground, just a fountain.