the adult is training kitten wrong. gently slip kitten off of child and set aside. That teaches cat that human is not a plaything. Poking at the cat like predator or prey, actually confirms that 'toy' instinct was right, and it will ingrain the habit.
ETA - You can actually see that cat was being super-gentle, so he was just sorta 'playing with baby' as instinct said, but you never reinforce 'human flesh' as 'cat toy' for any reason. just stop cold if it goes there. When they grow up they will love belly-rubs...