Road bikes would be perfect for 95% of the route. There are a few well surfaced dirt tracks. There was only one short stretch of flinty offroad that I thought was totally unsuitable for a road bike (getting close to Paris) but you can walk it in about 20 mins with no problem. If you’re on a budget a good mtb with slicks would suffice.
I’ve always found that side panniers are a bit like an anchor in the wind from both an aerodynamic and weight perspective. Fine if you have the strength, but for younger ones I wonder if they’d find it hard going on tougher days. Personally, I’d be tempted to be the carthorse and take the majority of the weight via a bob trailer and pannier rack but I don’t know how strong your lads are… they sound hardy.
If you keep yourself well fed, snack little and often, you might surprise yourself with the distances.
Hope that helps… Good luck!