I would avoid washing it too much as I'm cautious of how it may deteriorate the kite fabric, but I have washed mine with a small amount of car wash and sponge when it was new and I was a bit pedantic. Now I dont really see the need when unless you are kiting in muddy lakes as you say and it gets dirty. Rinsing with a hose should be good enough.
Whatever you wash it with, make sure the kite its TOTALLY dry before putting it in the bag, as punters have said, otherwise youll get a bag full of mould.
