I have seen many many kite notams but have never seen a kite (admittedly at my level, which is never below 2000ft enroute).
I did once see what looked like a plane-shaped helium baloon, painted in all kinds of colours. It was quite big (maybe 5m across). Went past me really close; maybe 20m away, so I saw it only very quickly. Farnborough saw zilch on radar... but then I know they can't see full size hot air baloons because I have reported those too.