If you want to be sure you don't ride alone, just check the following topics:
http://www.kitehigh.nl/forum/index.php?topic=21950.0 for Leiden en Den Haag
http://www.kitehigh.nl/forum/index.php?topic=23915.0 for greater Rotterdam
A lot of us kite either at the south point of the Maasvlakte (strand slufter, or "Da Sluf", "Sluf", "Sluftah" - I think you get it ) and Oostvoorne at a beach formerly known as "autostrand", now more oftenly referred to as "Playa", "Voetstrand" "Carplaya" etc.
If you want to be even more sure, just post a message here asking when and where people are going.
Good luck, cu out there,
Rudo
BTW didn't you post something similar on Hanglos a while ago?
Oh I forgot to mention that kiting at the Sluf will be over for a period of 5 or so years in a couple of months from now due to a major reconstruction of the Maasvlakte, it's said June but it might be later than that.