You're not asking for much, are you?
Look into Portugal. I'm not into flat water, but I found Portugal to be family-friendly, and great for kiteboarders. I think down south, in the Algarve, there are some flat water spots. Don't know much about them.
Up north, there's Viana, Esposende. Esposende has both waves and a flatwaterlagoon.
Around Lisbon, great city, also with kids, there's Fox do Arelho. Troia.
The only issue we had with the Portuguese coast is the fact that in most beaches the waves are too large and strong for kids. This is less of an issue in the Algarve.