took in two ferals, new cat owner.

So a few weeks ago me and my family have taken in two kittens, brother and sister. Everything is pretty normal, but I have a few questions. The first two are about the sister, Munchkin. When we got her she weighed only about three quarters of what she should and has gained weight back but is still not all the way there. My question is that her eyes always look so sad, like not open as much as they could be on the top half, do you have any idea why? I think possibly it could be depression that will go away as she gains weight or just the way she looks because her face is a little bony still. Also, her nose gets so dirty every single day. I read a little on it and it said it could be caused because of dust, but her brother Midnight doesn't have this. I also read it could be a symptom of a cold. She's eating fine and playing, drinking water and walking normal, everything else is fine. I usually use a Q tip with water on it to clean it but today I put peroxide on it instead and it started to bubble up like how it does when it cleans cuts. Is everything okay with her? I'm worried as to why it foamed up and to whether its normal or not. And with Midnight, nothing serious, more just my curiosity, but he makes this chewbacca sounding noise with his mouth completely closed and I don't know why. He makes it a lot when he runs and plays with his sister so I think maybe it means hes happy or anxious. Oh and he keeps doing this really weird thing! It's hard to explain, he'll just sit there and i think he rubs his tongue on the roof of his mouth and it looks like hes chewing on air and kinda sounds like he's chewing on something hard, but that could just be the noise of the roughness of his tongue rubbing against the roof of his mouth. I checked his mouth and there's no loose teeth or anything, he's been doing it for a few days. and one last question, does anyone know how to tell a kittens age by its teeth? I tried looking it up on the internet but i need someone to be more descriptive. They have the canines on the top and bottom, but not the tiny incisor teeth in the front between them, but they are coming a little. And they have what I think are called the molars and pre molars, I have been giving them cat food and wetting it to soften it because I think its still too hard for them but am not sure, should I be wetting it or is it unnecessary? Thank you to anyone that helps me.