Midnight is an arbitrary number to a five-year-old. A countdown is not — it is the whole appeal, and it works at seven o'clock exactly as well as it works at twelve.
Somewhere in the world reaches midnight every hour, all evening. Announcing that it is "midnight in London" or "midnight in Tokyo" at 6pm your time gives a young child a genuine countdown, at a genuine hour, with no pretending involved.
A number written large, ten seconds counted out loud together, something that makes noise at zero. None of that needs to happen at actual midnight to work — it needs a room full of people counting together, at any hour.
Handing a child the job of announcing it, rather than announcing it at them, is the difference between a countdown they watched and a countdown they ran.
It is a party trick a child will happily be in on for years. Nobody feels cheated by a countdown that made noise and ended in cheering.
That can run alongside — the younger one gets an early countdown and goes to bed; the older one gets the actual one later. Both get a real New Year's Eve.
Part of our guide to Holiday hosting with kids.