I think an otb tournament with 400 kids and G/5 would be organizationally a disaster, with most of time spent waiting between rounds for new pairings but go for it if you want to organize one.
With 900-1400, as our state event typically draws, the facility is everything. We use large convention centers which we rent for one day for $25K.
There is usually an hour between rounds one and two, and half an hour between subsequent rounds. In 2018 and 2019, we ran a tight schedule and kept it. Kudos to Rick, who doesn't even play chess, but who is a terrific organizer.
Most kids games without a clock look a lot like blitz with a clock.
I think an otb tournament with 400 kids and G/5 would be organizationally a disaster, with most of time spent waiting between rounds for new pairings but go for it if you want to organize one.
You can do them online with everyone at their own home but that isn’t the same type of experience as a communal otb tournament. I don’t think it will be nearly as much fun for the kids, but of course during covid options are limited.