Children...are like hurricanes. They start out as a Category 1 (newborn - 6 months), slightly disruptive, a little noisy but the damage is minimal. Then when they start crawling they move up to a Category 2 (6 months - 1 year), still disruptive, a bit noisier and the disaster area has spread further out but is still fairly manageable. Then there comes walking (1 - 2 years) which is a Category 3, where we have moved beyond disruptive to destructive, you attempt to contain the disaster area with a levy (or baby gate) but many times this barrier is tested. Minor injuries have been reported. Now we move on to talking and climbing (2 - 4 years) the noise level has reached that of a fighter jet and all logical thought flees the mind, evacuate all valuables, the levies have failed as the path of destruction widens and may be irreparable, injuries reported are serious but non-life threatening. Category 5 (4 - beyond), hopefully you've checked your insurance policy lately, run for dear life and take cover, injuries are life threatening, disaster area is all encompassing and demolished beyond repair. You can call FEMA but they won't come, it's too dangerous. This category lasts until they move out...Good luck and Godspeed.
Note: Any additional child automatically multiplies the destruction by 50%.