Nothing? You might disagree, but parents have the right to raise their minors how they see fit.

I guess you can picket the church?

Unfortunately, it's entirely possible that this sort of shit is legal, and then you can't do shit.

Follow them with some tupperware and scoop up the lesbianism. That shit can fetch a pretty penny on ebay.

Sadly, the most you can do without risking it with the law is expressing your disapproval to these people by talking to them, not that it will change anything.
You can make it dramatic, burst into the church, call them out on being insane, which will be a fun spectacle I assure you, if not effective.

Leave them alone. You can't just tell people how to raise their children no matter how shitty they are at it.

Call CPS on them.

Call some gay right activists on them.

fucking burn their church down

jeez, you guys are pussys