Using developmental psychology on your children

I do not feel guilty about using my slim (roughly one semester's worth plus another semester's worth of Ed Psych) knowledge of developmental psychology on my own children. After all, Jean Piaget, who described some of the stages of understanding children go through, developed his theories by watching his own children.

I am aware that three year olds have not yet developed the concept of conservation of mass (the idea that liquids don't change their amounts when poured between different shaped/sized containers or that say, play-doh doesn't change its mass if you change it's shape unless you remove some).
So when Doug was whining that I had poured too much milk for him and he couldn't possibly drink it, I merely poured the milk in a smaller glass and he was happy.