First, let me say that fenced in play grounds are AMAZING! Whether you have twins, multiples, a child with special needs, or simply need a safe spot for your one babe to run around - a safe and secure play area is a sanity saver for both parents and nannies.
I've compiled a list of areas in and around Seattle that offer fun fenced in play areas. Many of these areas are accessible, and help foster an inclusive environment. This is certainly not an exhaustive list, so please email me if you have a great spot to add!