I was one of those kids, and didn't end well for me in a lot of ways. Still sorting that crap out at 30.

Fundementalist parents are a huge reason young adults like myself leave Christianity and never look back. Growing up in a household like that pretty much alientates us from Christianity and leaves us with a serious grudge and potentially a lot of trauma.

"I just wish there was something I could do to help them."

Probably the biggest thing you can do is to provide an example of what a non-judgemental, loving, and overall decent Christian looks like. Love people for who and what they are, don't alienate LGBT+ people, and actually emphasize the teachings of Jesus. You'd be surprised how much the OT gets preached among fundementalist and Evangelicals. If I'd actually growing up in a liberal or UU church, I might not have the level of rage that I do and might still be a Christian. Ah well.