(NOTE: The following information was revealed in Season 11 Episode 92: "Reasons"). When Wyatt and Chris were 5 they knew each other, and they played together well. He did not know Danny, though. They used to be best friends, until Bart got a fake call that Chris pushed Wyatt down a sewer. However it was not true, but Bart thought it was. Bart told Wyatt never to play with Chris again, and there was a large feud.
Two years later, Bart found the guy that called him on that day, and found out it was all a lie. It was actually a friend of his that worked at a local supermarket. Bart called the cops and he went to jail. After that, since Bart is a prodigy at piano, he learned a tune that could make anyone fall asleep, or forget a certain memory. He used it on Wyatt and Chris, so they would forget everything about the feud. But Bart still didn't really feel too comfortable about Wyatt meeting Chris, but it had to be one day...
That's why in Bart's Son, Part 2, in Season 10, Bart reveals to Wyatt that he didn't feel too comfortable with him hanging out with Chris, but Wyatt didn't listen, thus, secrets were revealed.