You don't have to spill antifreeze or leave it where animals/children can get to it for them to get any and it be devistating.
Cars leak antifreeze all the time..go to any given parking lot (especially in the winter) and take a look around..at all the little green puddles (anti-freeze is generally flourescent green colored). A tablespoon, if I'm not mistaken, is enough to kill a small animal (like a cat, raccoon, opposum, ect.) in a matter of days (irreversable damage is done after only 1 day without charcoal treatment though).
As Synirr said, you need coolant in your car at all times, otherwise your engine will over-heat and it will expire..quickly.
Most people use water during the summer (which they really shouldn't..it doesn't keep your engine nearly as cool as antifreeze, which lessens the life of your engine parts) and use anti-freeze during the winter. It simply keeps the water in your radiator, and water filled engine parts, from freezing and bursting..which will ruin whatever part ruptures. If you drive your car with frozen water in the engine..not only can it crack your radiator but it can also crack the head gasket (due to over heating) whcih will allow water to get into your oil pan, which is very expensive and in most cases not even worth fixing (in older cars, due to the costs, because they have to tear the entire engine apart to fix it).
What we all really need to do is keep a check on our cars. When you pull out from your parking spot...periodically get out and check the ground to see if you are leaking any fluids. If there are any wet spots..find out what it is (you can wipe a papertowel on the spot and smell of it..if it has a sweet odor it's anti-freeze, if it has a more motor-y smell it's probably oil...neither are very good for the environment BTW) and GET IT FIXED!!!
Just because you keep your bottle of anti-freeze locked up away from YOUR pets, doesn't mean you are keeping ALL animals safe from it's effects...that puddle dripping under your car, your neighbors cars, ect...will kill just as readily.
Just some FYI.