I'm guessing that's a tricky question to answer, so I'm going to provide lots of details. Thanks for your patience!
tl/dr: I was out for a walk with all of my kids. My dog does well with kids (small children in large numbers visit us frequently) but less well with grown men. While walking by a yard full of kids, a 2 year old came up quickly and gave him a friendly scratch on the neck. My dog didn't seem to mind (or at least didn't react strongly). The child's father came up a moment later and picked up the child. A few moments later my dog gave out two short and low "howls" (I'm not sure exactly what sound he was making). I didn't notice anything off in his body language during the encounter. Any guesses what he might have been communicating?
My dog and people
My dog is effectively my first dog, so I'm not a dog whisperer by any means, and I'm trying to understand him here. He's an elderly (~10 years old) rescue, weighs about 75 pounds, he seems to mainly be American Bulldog, he's been with us for not quite a year, and he seems to have otherwise spent his whole life chained up in someone's back yard.
Despite that last fact, he is actually fairly well adjusted to people and does very well with small children. The "incident" in question happened on a walk, but he mainly meets new people while at home (he definitely has a "guard dog" aspect to his personality) so I figured I'd explain how that usually goes.
We're a larger family (5 kids, aged 0-11) and especially with the recent birth of our youngest, have had lots of people over in recent months, including many families that also have 4 or 5 young children. He does great with our kids, and has never shown any hints of aggression towards them, even when they accidentally hurt him (which doesn't happen much). He's always been very friendly with women and children visiting our house, although in the early days he would sometimes let out a low growl when first meeting new people in our home (typically while wagging his tail, sniffing them, and otherwise displaying a curious/excited body language). I would scold him for the growling, he would stop (or I would put him up if he didn't), and (in my non-expert opinion) he was never considering hurting everyone. Even during visits which started with a growl, he would be perfectly fine after a few minutes and would play with everyone - happily accepting pets and scratches from the new people just like our kids.
Over the past few months especially we have had even more people over (mainly because of people coming over to help with the baby), and during this time he's stopped growling at new people all together. The only time we have problems is with guys, which doesn't happen very often. I had someone come over to do some work on the house and the guy's son came in first (he was probably 18-20). For a minute our dog was fine like always, but suddenly started barking at him (I think the worker walked by one of our kids and might have made the dog uncomfortable). I put the dog up until everyone was done so that the workers didn't have to worry.
Out for a walk
The other day all the kids, the dog, and I were out for a walk and passed a family playing in their yard. This happens often in our neighborhood. Sometimes I cross the street in case I think our dog might make someone nervous (since he's on the larger side), but we're typically ignored by whoever is outside, and our dog is usually more interested in sniffing the grass, so I don't always.
This time there were about 4 kids, mainly playing on the sidewalk. They went back into their yard as we slowly came up, and I didn't think to cross the street (my kids had run a bit ahead, so mainly I was just following them) These new kids were in the grass just to the right of the sidewalk, and my dog was on the left side sniffing as he went along. There was a very young kid (maybe not even quite 2 years old) that I didn't notice, and he "broke away" from his family and toddled up to my dog quite quickly. The father was standing near by and told his son to stop, but of course he kept walking.
Little ones can be surprisingly quick, and a moment later he walked right up to my dog, reached up, and scratched him on the neck. The father came up a moment later and swooped up his son. I don't think I would have had time to do anything if I had thought to, although I wouldn't have been worried even if I had time to think about it. My dog just stood there calmly while the boy scratched him. The encounter was very quick, but I didn't notice any change in my dog's body language to suggest he was upset.
The father looked mildly panicked so I stopped for a moment to let him know that our dog is very used to little children, and that his youngest son was never in any danger. While I was talking my dog let out a low "howl" sound. I'm not actually sure how to describe it. It definitely wasn't a bark or a growl. He did it twice. Sort of a deep "how how" sound while tilting his head up a bit. A moment later we continued our walk.
I have never heard him do this before, so I'm not sure what he was trying to say. I'm guessing it was an expression of mild dislike about something in the encounter, whether it was the crowd of kids, being scratched without warning, or the father coming near him suddenly. I figure it's hard to say for sure, but I'd like to have a good guess so I know how to act in such situations in the future (aka just cross the street and avoid them). Any thoughts as to what my dog might have been saying?