I wouldn't encourage them too much to go to ground if there's nowt there myself, their natural instincts will lead them there when they want to go any way.
In the meantime, have them killing rats or other useful work and be patient, lets be right about it here, if they're 8 months old now your not really going to be digging to them this year.So another 12 months mouching around is going to do more harm than good.
Just my opinion mind, although in my house, my word is law, (when the wifes out )