My son has a life long fear of dogs, due to a dog chomping his head when he was just over a year old. We had 2 dogs (brothers). One was ok with anything small and puppy like, the other one was a pain with any child.
I had a clue that dog number 2 was not good round kids when he bounced at my older son and badly scratched his back.
Number 2 dog warned ds2 when he was eating his food and ds2 was crawling over to investigate. I never left them alone together before and was even more vigilant afterwards.
The 2nd he went for ds2, I was in the room, ds2 was pulling himself up by the couch near the sleeping dogs. Cue one surprised dog who lashed out and took a chunk out of ds2's head.
I knew dog number 2 was dodgy and to this day I regret not insisting he was found a home before ds2 came along. My now XH used to wind him up to the point where he would hide with me to escape it all.
Dog number 2 was rehomed with an older couple, with no kids, which I insisted on and they loved him from day one.
Looking back I wish I'd rehomed the X and retrained the dog.
We still have dog number 1, who is a grumpy old git with puppies, but ds2 who is 8 adores him and has fantastic control of him.