I once made a sandcastle cake and I used graham cracker crumbs but instead of worrying about sticking the crumbs to fondant I just used buttercream which worked well enough. Maybe you could make buttercream that matches your fondant color (or even a sand color) and spread that where you need to attach sand?
I have also seen people use brown sugar for sand but in my opinion if there is going to be a lot on the cake rather then just a sprinkle for decoration the graham cracker crumbs are much tastier in large amounts.
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