Uh, tell your son to figure it out and get them to stop or he'll be looking for another location for which to house his dogs?

Would you like to borrow my 'kitty'?

yes cayenne pepper it want hurt them spray on plant and ground they will catch on . then quit going there and digging. then you wont have to kill them ha ha signed tropical breeze

Chilli pepper, ammonia on cotton balls or small containers, and vinegar on cotton balls or small containers.
I also like the idea of a new home for the dog LOL!!!!!! My prob in CF was squirrels digging up any bulb I planted. Nothing worked!!!

restrain the dogs to a specific enclosed area? like a kennel or run.

OR put up fencing around your beds

OR remove the dogs from the area.

OR put the dogs on anchored leads in the yard out of reach from the beds.

OR get some device for training them and sit out there waiting for them to dig and then deploy the training device (loud noise, loud verbal commands, water blasts, shock collars).

Dogs can be trained not to do this type of behavior but it takes time and real effort on all parties parts. I recently saw an episode on "it's me or the dog" on animal plantet where the lady trained dogs not to eat their own and the other family dog members pooh. She reinforced them with treats when they obeyed a command to "leave it" which was by using little flags in the yard. She'd place the flags at the pooh site and then command "leave it" when they left it alone, they got a treat. She started out just using the flags other places with the dogs, so that they would understand what they were being trained to "leave" which was the flag association. then it became related to the pooh as well.

For bulbs, could you lay chicken wire over the area and cover with a thin layer of dirt? The bulbs could still grow through, but the wire might make the digging less satisfying for them. Of course, this might not work well in an area already grown up with other things.