I have a 7 year old girl who is in her 3rd season. When the littles say they're done and want to go in for the day, go. Don't try to make one extra run. Always end on a good note while the child is still in a good mood. If you have a really young one who just wants to play in the snow rather than ski...let them. They have plenty of years ahead of them to learn to be an awesome skier.
Give LOTS of positive reinforcement and try not to correct every little thing. Try not to make a big deal out of falls (unless there is an injury, of course). Try to stay behind your child if at all possible because, like someone else said, it's MUCH easier to ski down to a child that has fallen than it is to hike back up. It never fails...the second I decide to lead my daughter down, that's the time she'll fall and her skis will be everywhere.
Carry small snacks in your pockets (granola bars, etc.). These can be lifesavers for kids who get hungry between breakfast and lunch. Carry hand warmers on cold days for cold hands.
Keep it fun! If the child is not having fun they'll be much more reluctant to go out the next time.