To start just encourage empathy, generosity, collectivism, and a desire to learn. Eventually when they start to notice liberal programming just simply debunk it. When they here about billionaires in the news or from peers tell them about how much exploitation, greed, and luck got them to have that wealth. Point out how many better things could be done with it. If you see homeless people in the street tell them about the injustice of the system that brought them there. How there are many homes that they could live in, but the landowners are too greedy to let them. Just frequently talk about social issues in your household and the children will listen and try to understand, because as a parent you’re interesting to them, they might even ask questions. I could go on for a while, but I hope this helpful.

PS I’m actually a teen and this is what my socdem parents did for me. I’m sure the values they instilled are a major reason why I’m a commie today.