i was just blathering on in another thread and got curious.
At eighteen months, my daughter:
Helps fetch in firewood from the carport and puts it by the combustion heater
Helps unload the dishwasher, under strict supervision and I remove sharp knives first
Feeds the fish (and would happily do so 186,386,594 times a day if I let her)
Helps tidy away toys
Hands me pegs when I'm hanging out washing
The only one of these that is more helpful than doing it myself is the firewood, since she'll carry a log while I'm carrying some and it gets done marginally faster. But it's a start.
I suspect by the time she's competent at housework she won't want to do it anymore, but what do your children do around the house as standard?