Have you seen blowkarts???
www.blowkart.com/
Avalon Sail co. are the agents in Mandurah.

Are you thinking of windsurfer style rig or land yacht???
I think skate boards are a bit too small to be ideal.
Most hardware stores sell small wheels of different sorts.
The biggest problem I see if you want a windsurfer style device is foot steering it.
You can modify a skate board style suspension to be much more manuverable but then they become liable to speed wobble. I think it's the front trucks that have negative castor, may work if you only modify the rear trucks, depends how manuverable you want it.

I have a home made landsailer that works excellent. The best wheels are definately the golf buggy wheels that have inner tubes. You can use it on hard sand, grass, gravel and the road. I used to use mine at an oval after school when I couldn't get to the beach.

Mr Greenroom, let me clarify. I am talking when I was very young. The waves at Perth ARE suitable for wavesailing when your 4 years old. Actually, when using my landsailer on the cricket pitch it is very similar to sailing the waves at perth.

Thanks for the replies. I want practice my gybes and tricks in the park accross the road on the not so windy days( kids can run behind me as well). I have found some wheels on ebay so I'll let you know how I get on.