my white cat had skin cancer on her ears and had to have them amputated. She lived for several years after that, but I would always advise anyone to put suncream on their cat's white ears.
Seriously, tis a recognised problem.
The problem that we had was she would be out all day, so I would put the suncream on and she would disappear for the day, so she still got skin cancer.
It is ok to use kids' sunblock - as high a factor as you can get. Reapply it as often as you possibly can, you can't overdose them. Get a waterproof one if you can as well, because the first thing they will do is try to clean it off.
You really, really, really must do this, forget everyone who laughs at you. I will send you pics of my earless cat if you want something to shut them up.
(and my poor mog went and had an accident and had one of her legs amputated as well - it got so that every time she went to the bloody vet they chopped something off. dp said we should have named her "lucky" )