When they are puppies, they are exploring their world with their mouths. I agree with Ringo, you have to watch them like a hawk. When they go to lick or grab something bad for them, use keywords and phrases to train them that it is bad and then move them along.
Max had a thing for bird droppings. Every time he went to lick one, I would say "Yucky, that's not good for you," and then I would move him along. He responds very well on the leash because he knows he is going to get "helicoptered" away from the yucky thing if he doesn't listen.
Off leash is more difficult. I inadvertently trained Max to grab things like leaves and run to his naughty place to eat them. He thinks it is a game because I always chased him. It's hard to keep them safe and train them at the same time to give up the bad thing. When possible, teach your pups to trade the bad thing for a treat.
Hang in there. It usually gets easier with maturity, but you will always have to be vigilant with these little ones. There were times when I was exhausted keeping things out of Max's mouth, so I would put him in his xpen for 10 minutes just to take a break.
