Many dogs love to dig. They often dig when they're bored, and they can often be bored! "Day" is a long, long time for a very young dog.
This is what we have done: We've put a fence around the flower beds - a fence high enough (at least a couple of feet high) to discourage a dog from jumping over and having fun in there. For little random holes - the "No, ma'am, I don't know *how* that hole got in the yard, pant, pant," kind of holes - we fill the hole back up and then apply a layer of red pepper. It usually works.
We keep toys and bones outside so the dog will have things to do. Out in our yard right now are a couple of large balls for running around with. One is made of rubber, has a rope through it, and the other is a large, heavy plastic ball with a smaller ball inside. Google "Jolly Balls" and see what you find. What bones you could use depends on your dog and on what your vet says. We have those heavy bones with filler inside them, which I get at Petsmart. When our dog has licked the "stuff" out of a bone, I refill it with peanut butter and freeze it. The bones do wear out, but they're sturdy enough that I don't have to worry about their splintering and injuring our pup - but then, I'm not leaving him as long a time as you do.
You could think about another walk for your dogs in the evening.
We also used to have a "Rover spot," which our dog could use to dig as he pleased. Often, in the summer time, he'd dig himself a shallow depression to lie down in and keep cool. I would recommend that for you - a corner of the yard that nobody else will worry about. As I said before, all day long is all day LONG.