Don't rule out Middelburg, beautiful small city. And go see the "Deltawerken Neeltje Jans". The big waterworks built after the flooding in the 50s. They also have a museum where you go inside the dam and see the inner workings. Maybe a bit too overwhelming for a 2 and 3 year old though
Cheers!
That's a great base for me to start.
I've found out we're going to Vlissingen.