It sounds like Samson is going through puberty. Becoming a grown up bulldog. And, with another puppy around, it sounds like the family structure is changing, with even bigger changes to come.
Does your daughter feed Samson? Teach him? Is she "in charge" of him? Maybe you could practice letting some of the other children do his tricks with him. Dogs naturally feel superior to little people (they are the weak ones, the dog is strong) and unless you actively remind him that children are in charge of him, ALL children, it will be easy for him to mistake the other children (that don't have authority over him, neices and friends) for outsiders beneath him.
I also had dog once that didn't like short black men. For whatever reason, she was fine with every guys of every other color, height, appearance. We got her at 4 or 5 years old, so I'm not sure where it came from, but, maybe Samson has had a bad experience with a little girl? Paisley doesn't like hats. Not aggressive not like, but, shys away from people wearing them. Dogs notice the small things we don't even notice.