I agree with the idea of the beaches in Holland. We just got back from a week in Katwijk, which is right on the North Sea with wide, sandy beaches. We have done this trip a couple of times and always stay at the Noorduinen campground -http://www.noordduinen.nl/uk/index.html - we rent one of the little vacations houses and absolutely love it. The campground is within walking distance of the beach and the people are super friendly. Plus if you get tired of the beaches are you just a quick train ride away from Amesterdam.
Another choice if you are military is Camp Darby, Italy. The base is near Pisa and they have a private beach that is sandy and pretty kid friendly. We went last year and it was warm and calm enough from my three year old to splash around. The lodging on base is OK-a camp ground, some cabin-type things, and then basic Army type lodging. The website is http://www.livornomwr.com