YANBU.
My PIL had two very well trained, docile dogs. One of them was elderly (hence the 'had' - it has died of old age now).
My DS, aged 2 and a bit, was crawling up and down the hall with his twin sister, when the very well behaved and docile dog lunged at DS and pinned him to the floor with her jaws open wide around DS's face. This was all within 6foot of PIL, DH and me, so definitely not unattended, and she showed no signs of irritation before she lunged at him.
In hindsight (and boy do you relive it over and over), she was annoyed at these two boisterous 'puppies' in her space, and she put one of them in his place in the pack. She was an elderly Golden Retriever, with a soft mouth, and she didn't intend to hurt him, just tell him off for annoying her. Of she'd wanted to bite him she most certainly could have.
PIL made sure that the dog was locked in the kitchen/garage whenever we visited, until the dog died. I made a point of taking DS in to see the dog every visit as I didn't want him to be scared of the dog (I scooped him up from the 'attack' and tickled him, making it all into a joke that the dog had tickled him at the time too), but we respected the fact that the elderly dog didn't have the patience for toddlers in her space.
Please do keep your DS and the dog apart. You never know the moment, and the consequences are so big it's really not worth the risk.