I'm 52 years old and I did this when I was a child. My family nickname became Chipmunk Cheeks or Cheeks and stayed with me even after I stopped doing it. My mom can't remember when I stopped, but she says I didn't do it for too long.
I didn't get cavities, have no food issues, and I look back on that time with memories of security and enjoy the fun we have today laughing about it and telling our grand kids. Kids do the craziest things and some times it's just kids being silly. I think you'll know if the issue becomes one to elevate and get help. Good luck.