All the writers have to do is have someone mention casually where Holly or any of the other kids are when they are not obviously with their parents (on a sleepover, with grandparents, aunt, uncle, etc). They did this when Bo was in a coma and Hope was off to Alamania with John that time they were getting a divorce (when they found out they were married when she was Gina and he was the Pawn). Nobody in town mentioned where Ciara was, who was a small girl at the time. It wasn't with Doug and Julie, as they had went out of town at the time, so by default it was probably Caroline, but it was never mentioned onscreen.