I know with Snazaroo, if you only use their makeup, you are insured automatically. I think it's rare that a face painter only uses one brand and in that case there are several different routes you can go to be insured. Here are the ones I know about: The World Clown Association. You don't need to be a clown either, but you do have to have a membership. Here's their website...
www.worldclown.com ( I think this is the cheapest route and it's what I started out with.)
Specialty Insurance. Their website is
www.specialtyinsuranceagency.com. This is who I get my insurance through.
The third option I've heard of is
www.clownsofamerica.org. The prices vary, so you'll have to check them each out.
In regards to your customer having a reaction, I'm surprised it happened at the first event you used Snazaroo at, because I really think it's a rare thing.

I wouldn't think the sign would be necessary, but that's just my personal opinion. I've only had one incident where a boy's face got blotchy red from a Starblend powder, but he didn't complain about itching.