So, I've talked a lot already about how Strange just sort of slots people into distinctions. This is definitely important for Susan because he views Susan as a magician over a kid. Susan loves magic, focuses on magic, makes magic so much of himself that of course he counts as a magician and, as such, Strange will treat him as a magician. This means that Susan's a great person to talk shop to and gush about magic with...but this also means that he'll occasionally forget that the twelve year old is literally twelve and fuck yeah let's have you jump off a roof to try and fly, what's the worst that can happen.

The Susan/Lauren and Strange/Childermass parallel is definitely something that like...while Strange doesn't consciously realize it (because he's dumb), it definitely also affects his friendship with Susan. Because hey, he knows what it's like to have someone looking over your shoulder, yelling at you when you mess up, criticizing your spell choices, and not knowing how to people in the slightest (and yet you're good friends all the while). It really sucks. As such, Strange sees a lot of himself in Susan and is perfectly happy figuring out things and finding the best way so that Susan can cut loose and practice his magic and just love it the way that Strange does. Likewise, if Susan were to ever screw up and set something on fire, Strange would be the first person in line to defend him.

also, Strange already KNOWS he's written the book on bad ideas. Whatever ideas/plans/etc. Susan manages to come up with, he'll at least hear them out because hey, whatever it is, it's probably a better idea than he has.