Are you aloud to ride atvs in the sand dune kinda area

You can't in the sand dunes at the park/next to the lake... But you can off H loop and nearby.

I seen kids using snow boards on that sand patch bye the lake

Yes but only in October anytime before that you can ride from H loop but after that you must go to the day area and unload your bikes

Only in a special area

I have seen them ridden there before