Kirkwood has TERRIFIC areas for a picnic. There is a lot of shade and one of the play areas has a water spout thingy so if the kids get warm they can cool off again. The only bad thing is that it isn't great for kids over say...6 during tot time. They only have the toddler area open, but the big waterslides, regular swimming pools, and other things are not open. Even the bucket on the toddler area is turned off. The small water slide is open, off the toddler equiptment and the lazy river, around the whole thing, is open.
Personally, I just remembered that my son has swimming lessons for the next two weeks at 11:00am at our pool, in south county... so I wouldn't be able to go to tot time unless we did a weekend or wait till two weeks from now. Bummer. Can anyone do Saturdays?
Another option is Springdale, I think its out on 141, near 21... its a HUGE gradiated pool, so no one has to go in deeper than they want to. Picnic areas there too... don't know the cost though.