Ulcer under tongue

Last week I noticed what looks like an ulcer underneath my child's tongue. There doesn't seem to be in any pain. There's been no blood or swelling. It's just white and almost looks like the frenulum (?) has torn. I've included the best picture I could get. The dr said it could be a very odd presentation of hand, foot and mouth disease but to wait a week and see if it goes away. That answer didn't really sit well with me so I'm wondering if anyone else might have any insight as to what this might be.