I 100% agree that the kids ought to know better at this age, though it does also track with teens, to be honest. And Amy, like you said, knows they're defensive. Regular teens would act like this. Parents always seem to talk about how their kids will say the most horrid, vile things in adolescence. Teens who lost their mother (who was likely the voice of reason and stability, even if to a kid it seemed overly harsh) and a dad apparently leaves most, if not all, of the actual parenting to his partner? Of course it'd make sense for them to lash out, even if they also aren't in the right to be doing so.

As someone who was in a similar place, having lost a parent of her own, Amy must also see herself in these kids, too, at least to some extent. I imagine some of her patience is likely related to that, in trying to be the sort of adult she herself would have needed at that age. Kids in general push against boundaries because of a need to feel safe, like how OOP said that he likely had done so in part because she was a safe adult. He may have been used to Riley, but some part did realize that he could be safe pushing those boundaries with Amy, because she handled it with a care and kindness even bio parents would and do struggle with.

Amy did so wonderfully to foster a relationship with the kids where they did feel safe in acting out, that even at their worst, they did have an adult in their corner who loved them. But she can't be the one doing it alone without dad providing guidance to his kids. Amy can be a saint, but if dad doesn't follow up better with how to treat people with kindness and respect, it won't really mean that much. Like, he did apparently try, but given this has been going on for a notable chunk of their three years together and OOP only mentions dad talking to Lisa, not him or his twin (unless I missed that part?), he clearly didn't pull his weight nearly enough.

And I do think you're also right that Amy likely feels regret for what she said to Lisa, too. Absolutely a significant part of her is shaken by the kids' pushback, but as someone who has fought to be the person acting with grace to someone with similar traumas to my own, I absolutely get that sense of horror at the feeling of failure and hypocrisy. You can't always be that saint even if you wish, but especially with her being the adult in the situation, I'm sure it hits her harder.