So there have been cases where I've heard of people doing their child requirements before finishing their adult requirements. I met someone on an interview who was doing that. Mostly because of book-keeping issues in the department. I think there weren't enough child spots for everyone in the residency class that wanted to do child, so someone did their child a year or two early, then came back to finish their adult requirements.
Apparently, it's possible...I'd assume anyone from another specialty doing this would have to finish their adult psych requirements after the child ones, so it'd still take 4-5 years.