I dont know of any websites, But when i do kids parties i keep it simple.
Kids love glittery varnishs and mood changing colours go down great.
Try things like small daisies done with a dotting tool, Paw prints, Animal designs done with a striping brush,Kids love all this sort of stuff.
I do a lot of parties and if you want i can send you pics of some of my more simplar designs, ie winnie, eeyore,piglet,hello kitty,paul frank,burberry,union jack flowers hearts etc etc., with instructions. pm me you email address.