Why do transphobes automatically assume that if a child is trans under 10 they were groomed into being Trans/taught about lgbtq?

I often will see posts sometimes on FB, where a kid around maybe 4-6 will say that there trans and then all the comments are saying, “Child abuse” or “a kid shouldn’t be taught about that” stupid stuff like that and I just wonder why is that? Why do they think being trans is something 18+ and just assume the worst. I’ll never understand the brain of a transphobe..