Khaila could not have known that Isaiah "liked to play with blocks", since she had not had him with her while he was growing up from infancy, and he'd always just sullenly moped around whenever he was at her apartment and never wanted to play or interact with the other children at day-care, so Khaila would never have seen Isaiah when he was truly just "at play" and thus learned what toys were his favorites.
At 00:15:59 There are pictures of 4 year old Isaiah on Margaret's desk in her office at a point in the movie when Isaiah was still a newborn.
When Halle leans over to get the child out of the sandbox, her microphone pack can be seen at the small of her back under her shirt.