Keukenhof 20th of March to 17th of May, so unless you define summer differently than most unfortunately you will have to miss that one (copious tears now flowing)
The other popular windmill adventure is Zaanse Schans Don't be fooled by the true panoramic photo on their website which make it look like you are miles from everywhere. When it scrolls back across the bridge you don't see that just behind you is a built up industrial area and the train station. That's not wrong, they took it from the best angle - just like real estate. Also that's not to dissuade you, just so you are not surprised. It is a fun, very close to Amsterdam and therefor very full of tourists, very informative place. You can even watch clogs being hand made in your exact size.
Kinderdijk in the rural countryside east of Rotterdam really is surrounded by countryside, and the distances between the windmills is somewhat greater. You can see them in their natural environment, and you can even take a boat there if you wish.
I find the only real thing tying the two places together is that they both have operating windmills. For me they are like chalk and cheese, and I love both of them.
Did Tim's post mention Edam (with the floating basement) or Enkhuizen? Or Arnhem? Or Het Loo?
You could easily spend a month in the Netherlands and not retrace your steps.
Did anybody mention castles? Do the kids or you have any interest in real genuine castles?
Did you know that the Netherlands has mountains? Well, high hills that the Dutch are convinced are mountains. (Limburg).
BTW - Mom - I see that the kids are under 20. Does that mean 18 and 19 or 10 and 11?