If the kid has them, I doubt they'd be surprised. Just as you're not surprised when Wilhelm does something very teenager.

Istredd knows Jack hasn't stopped being a topic or a pain point, but Wilhelm is Lucifer's true parental relationship, and it's a positive one. Even with that hiccup, it turned into something real and strong. Istredd still doesn't entirely understand teenage behavior, his own rearing so specific, although he and Kell have created a stronger bond than he expected he could have. It's a series of feelings he's working on accepting.

True, we were already in each other's pockets before that, but this accelerated it. Their closeness began before the bond, that simply cemented it in an undeniable way. But they found a harmony as roommates and friends first.

I don't read fiction but I'm going to try to read this one.