As a new mom in 2003, I did not know a lot about the facilities at all of the area parks. I was thrilled to learn about all the wading pools in Seattle last summer when my daughter was turning 1 year old. I can't wait to take her back to them this summer.
The website has a list of the various wading pools around the city. They are clean and well-maintained. There is a "lifeguard" at each pool - seems like usually a college student working a summer job. Swimmers who are not potty-trained need a swim diaper, and the staff at the pool do a good job making sure everyone has one!
Visiting the community pool saves water you might otherwise use filling your own pool or running a sprinkler for your kids, so it's good for the environment and salmon too.
Take sand and bath toys, arrange play dates there, take your PEPS group but do check them out. Many open later this month.
My only "problem" has been that they don't fill the pools until noon, so it's not a morning activity. Plus, the staff won't let you in until the pool is filled, and it's hard to keep a determined toddler out of the pool at 11:55 am while it's still filling!
(ps, if you're in the Northeast end of Seattle, Viewridge and Ravenna have great pools, but we like Dahl too - this little park is not well-known and the pool is never crowded - but shhhh....don't tell anyone else!)