Fence off a small area around the box for them to play and rest in. Most likely they won't "go" in the only place they have to play while you're gone. When you get home let them play in the rest of the yard as well. Basically, don't let them have 'run of the yard' while you're away. It's the same if they were inside. You'd only let them have one room or a hallway to themselves so they wouldn't tear up or soil every place available. They are small dogs in a big world, everything is theirs for the taking (or pooping) if it's given to them.