At some point the puppies have to see, smell, and learn to bay a live critter in a controlled situation.
I normally use squirrels for this as they are easy to catch, and if they are a little freaked out by the experience of being so close to a dog, I am more than OK with that, as my yard squirrels need to learn to lay off my bird feeders at least a little bit.
I set the box trap Sunday night, as I often catch squirrels early in the morning, but in this case it was a possum. We don't get a lot of possums, probably because we have so many fox, plus neighborhood dogs are hard on them. In any case this guy was easily transferred to a pet carrier, given a meal of grapes, sunflower seeds, and raw peanuts, and set aside for a little after-work dog training before he was let go right where he was caught. He was very inanimate quarry - I much prefer squirrels. Maybe tomorrow with them.
The dogs did about as expected for their first time seeing quarry -- they sniffed, walked around the crate, and then began to bark, quickly building up confidence as they came to understand that this was OK in this situation.
Two or three more practice runs with squirrels, and the penny should drop enough that they will be ready to start digging through mulch and dirt in the tunnel in order to get to the squirrel at the other end of the pipe.
Slowly, slowly, we give experience and build confidence and brain power. These are very young dogs. No one ever ruined a dog by entering them too slowly, or a month late.
A little kibble tossed into hollow logs, trees, and holes teaches them to explore to find rewards. Soon the scent of quarry will be all the reward they will ever want or need.