I think you need to find a new trainer. A trainer might suggest a muzzle if you are trying to actively train him and you want to prevent any bites during a training session, but it's not a method of daily control or punishment. If he doesn't like strangers I would remove him from the situation and let him retreat to his own space.
For the resource guarding, you need to teach him commands like "drop" and "leave it" Loki used to guard food toys like Kongs and so I taught him "drop it" and I can just walk up to him and say "DROP IT" and he spits it out into my hand no problem. Since he knows (from training) that he either gets his toy right back OR a *better* treat in exchange, there are no issues any more. In your case you would also need to train each KID how to do this too.
"Leave it" is great too and easy to teach. There are some great books by Pat Miller and others that explain it, plus I still think you need a new trainer.
Oh, as far as the bed, you can let him on the bed as long as you *control* the bed. That means he gets off any time you ask. Work on the "OFF" command. "Move" is great too. Mine sleep on the bed, couch whatever but if I ask them to get off or move they comply
