Praise them when they do something you want them to; make sure they're well fed; their sicknesses are taken care of early; buy them groceries at least once a day whether they need them or not; give them candy or fruit when they do something you like; stay away from that nasty scolding glove...positive influence works much better.
For the kids; you can have mom/dad tickle them; read a story to them; buy them a movie to watch together as a family; give them candies; also you can have the kids play train together or have a giggle party; and like the adults, groceries and upgrades make them happy.
Also, if the house if not kept clean it will cause them to become depressed.