I'm the original poster on this thread and it's my kid that has caught leftism. I wanted to try to provide you some insight because your question seems genuine. Here's our story and what I think makes the difference.
I have 2 children, both college age now but they were born 5 years apart-same father. The first one was born just in time to be a part of the millenial generation. Second was born at the beginning of Gen Z. As we know Gen Z is already proving to be more based and more conservative than the millenials.
Millenial child always showed signs of being temperamental, was and is very smart in a book sense way but always lacking in common sense. She attended public school from Kindergarten through high school graduation. Again, very smart, never seemed to have to do homework because she had already done it at school, always straight As from K-12. The only problem I ever had with her in school is she had trouble making friends and she felt compelled to threaten a classmate in 8th grade that if he didn't leave another person alone she would "put a foot up his ass". She wrote this down. Obviously they turned it in and she was suspended for communicating threats. (see what I mean about lack of common sense?) When she was 7-8 her father and I split up, she remained in public school and eventually when she was no longer required to be in after school care she spent her afternoons at home alone until I got home from work. She spent that time online. At first I set up restrictions on the time limits but honestly, after lots of begging and pleading from her I softened on that as long as her school work was getting done and her grades remained good.
This was a mistake. She began browsing role playing forums. Ok, no big deal at first. She liked to draw and is a gifted artist. She was obsessed with wolves and the role playing forum she spent her time on was about wolves. I cannot even tell you how many different sketches and drawings of wolves I found in her room when I packed it up but it would number in the 100s. Eventually her role playing moved on to furrie stuff which I admittedly should have investigated more but I didn't. I mean furrie sounds innocent enough, right? But no, furrie culture is not innocent. I think this is where the slow slide into moral degeneration began. Then she was introduced to reddit. I knew nothing about reddit. Now that I've seen it I wish I had blocked it from her ever being able to reach the site because it is full of leftist brainwashing and moral degeneracy. In fact, moral degeneracy seems to be revered on reddit.
Ok, this is getting long so I'm going to wrap it up by saying my youngest child, who is quite the opposite of her sister, was home schooled, had very limited internet time and was encouraged to attend church with her parents much more than oldest. If your children must go to public school monitor their homework, see what they are teaching. If you see something they are teaching that goes against your principles talk to your kid about it. Take a stand at the school (especially if you see something like that person posted the other day where they are having the kid list the 5 pillars of Islam and such). Monitor and limit their internet time.
FB, Twitter, Reddit, Tumblr are all places where indoctrination happens. Keep your kid off of them if at all possible. Instead encourage playing outdoor sports, camping, fishing, hiking, family outings, etc. Discourage hours of internet social networking.