Hey K.!
My daughter also does this. She has done this since she was about your daughter's age. But to be honest, it wasn't as bad until she gave up her pacifier. (Which was much later, I have to admit.) Now, it can still be bad at night, depending on how her day went. Stress plays a big factor. I am taking her to see an orthodontist soon. My dentist didn't see anything wrong with it. (We have since switched dentists.) But his hygenist did. (She has since quit this dentist...long story.) She told me her son did it and they had him fitted with a mouth piece. I don't know that they could do this for your daughter because she is so young. I wouldn't normally recommend this, but until she gets older to be fitted for a piece, maybe give her a pacifier? My daughter chewed on hers and it made it not so bad. I know it isn't the "ideal fix" but it is a temporary fix until she gets more teeth.
***Another thing I do when they sleep, because I can hear it across the hall because she does it so loud, i whisper in her ear...Zan, stop that, honey. And I rub her jaw. She wakes up enough to quit and then goes right back to sleep.***
But no worries! She did this as much as your daughter and she still has all her teeth!!! She just now at the age of six lost her first one! But this can't all be fixed.
Take care and let me know how it goes! ____@____.com
Lee