Robbie is the young child that appears in Paranormal Activity 4 who appears to have demonic powers and able to see Toby like Wyatt.
No information is known about Robbie after he was staying at Alex's house. However, Wyatt said that Robbie was adopted by Katie. After Robbie met Wyatt, he eventually became friends with him, which apparently, he also brought Toby into his own house in which the three of them later become friends.
Robbie disappears about halfway through the movie.