ok this is going to sound like a dumb question, sorry. Are you sure the kitten in panting and not actually trying to meow without any sound coming out?
Everything I have ever learned is that cats should not pant or "open mouth breath"...ever. It is a sign of something serious being wrong in most cases.
The fact that this kitten does it when Mom is out of the box, thus not likely due to overheating, makes me wonder if she is actually crying and not panting.
We have a kitty named Mute, because he did this until he was almost two years old. Then all of a sudden he found his voice. It is not so funny anymore since he "talks" all the time. Wasn't so bad when nothing came out.
If it really is panting, I think at least a call to the vet is warranted.
Sending lots of vibes that nothing serious is wrong with the little one.