For a business it would be considered a universal waste. For residential, take it to a recycling or waste reduction facility. Many bigger cities have hazardous drop off sites.
Everyone knows mercury is toxic to humans. Don't play with liquid mercury. Don't call a poison control line with a false emergency to find out.
Elemental Mercury is not a problem. Just call any Poison Control Hot Line and tell them that your child has broken a mercury thermometer in their mouth. They will tell you that the elemental mercury will not harm them and it will pass in a couple of days! It is Mercury Vapors and the Oxides of Mercury that are dangerous! I have played with liquid mercury at room temperature all my life.