A way to build rapport with a child is to jump into their imaginative play, make up a fun game only you play with them, and reinforce with anything and everything. With one of my clients at my clinic, he enjoys tickles so I change the way he gets a tickle (i.e. crab claws, jellyfish, shark, alligator, zombie) switching it up and making it more fun for him and making myself a reinforcer for him. He only gets these types of tickles with me. Finding some activity real or make believe that they only access through you.