Father Abraham - Lincoln and his Sons

[Donna McCreary]

"Good evening Mr. Lincoln!" he announced gravely. For this, in front of all the guest, "his father gave him a gentle slap in the face". Lincoln and his elder son clearly becoming strangers to each other."

Robert was making a joke. It was a gentle slap (like a loving pat) not a slap made in anger. I do not read this to indicate the relationship was growing distant, but that they had a closeness and teased one another.