OK, this is actually what I do for a living, so here's more than you wanted to know about antifreeze...
The problem with antifreeze is that it contains ethylene glycol (EG). Diethylene Glycol is found in some brake fluids and some manufactures put a little bit of it in their antifreeze. Methanol is found in windshield wiper fluids amongst other things. Some antifreezes do contain methanol but it's not very common. The concentration of EG in antifreeze is usually about 95%. For the sake of this conversation, we will assume antifreeze that is 95% EG and ignore the other components for the sake of simplicity.
The metabolites of EG cause kidney failure and metabolic acidosis (lowering your blood's pH). Either will kill you eventually without treatment, but the metabolic acidosis will kill you a lot more quickly, so that is usually the cause of death. If you survive the metabolic acidosis, you may live to experience the joys of needing dialysis 3 days a week for the rest of your life.
Now, as for an amount that will cause problems up to and including death. We usually worry about a blood concentration of > 25 mg/dl. Some examples:
For 95% EG in a 70 kg (154 lbs) person, that would be about 14 ml, or half an ounce. So, is tasting a drop off your finger going to be an issue? No. Is taking a swig from the gatorade bottle your redneck brother-in-law uses to store his antifreeze going to be an issue? Probably.
For 95% EG in a 10 kg (22 lbs) toddler or small dog, that would be about 2 ml (less than half a teaspoon). So, if a toddler licks it off his finger it's probably not going to be an issue, but if they drink it from a container or even slurp it off a garage floor, it's gonna be a big problem.
The good news: with prompt treatment (getting to an ER within about 6 hours of ingestion is usually sufficient) EG poisoning is rarely fatal. There are actually 2 antidotes available (good old ethanol being one) and with appropriate treatment, people are usually discharged in a few days with no permanent complications.
Important advice: Don't try to figure this stuff out on your own. If you have reason to believe you or anyone you know has ingested more than a taste of antifreeze, call your local poison center at 1-800-222-1222. If you think a pet is at risk, call the national animal poison center at 1-888-426-4435
Sorry for the thread drift...