I know it's easier said than done, but have patience. Keep trying the live hopper mice, but I would give it 3-5 days between attempts. If they're not taking live, it's very doubtful they'll take f/t. Once you get them started on live though, it's usually pretty easy to switch them over to f/t. Once they get started it seems like they'll eat anything, any time. Other than that, leave them alone.

How long before worrying? Tough call. Hatchlings can go a fairly long time without eating, depending on their condition when they hatched. Odds are, they will all get with the program and start eating. You can monitor their weight, but I find it just as easy to keep track of their condition visually. If they don't eat on their own, you would eventually have to look at assist feeding, but I imagine you've got some time before that becomes an option.

Good luck and keep us posted...