Well, when you lick mercury, you're actually going to swallow a lot of it. Thankfully, you'll poop most of it out, and as long as you do it once, it won't kill you.

But if I had to pick between licking lead or mercury, I'd go with lead.

look at this shill from the big metal licking industry

It isn't safe at any dose but the amount of harm from licking it once is definitely rather small. Probably safer than having a couple of alcoholic drinks or a single cigarette.

I have a toddler and I hope to dear god there's no lead about. She will lick anything.

The inside of your mouth is on the outside of your body and so is the rest of your digestive tract, safely (well) isolated (unless it's permeable) from your bloodstream. As a first approximation, our bodies are toruses. Just licking something is really not much more intimate than touching something unless it's sugar which can be taken up right there on the spot. (At least glucose can and there's enzymes in salvia don't ask me for details).

All that said the Romans used lead(II) acetate as a sweetener and while definitely a bad idea, they didn't all immediately keel over either. You'll almost certainly be fine.

Pure water OTOH... you'll burn your mouth because osmotic pressure tearing cell walls apart before the stuff dilutes to have a sensible amount of minerals in it. The tissue there is used to sudden violent cell death and heals quickly so no biggie, if you survived a too hot pizza you'll survive water. Also, do eat that pizza to have enough minerals to replenish everything.