Welcome to Yorkie Talk. Are both of the dogs neutered?
You are wonderful for walking the dogs and taking such an active interest in them.
I have 2 boys who are pad and outdoor trained. The older one has been a leg lifter from the time he was a pup, although he always went to the pad. For him, moving the pad away from vertical surfaces solved that problem. He still lifts a foot, but doesn't do a full vertical lift.
Praising and rewarding for going to the pad I believe has kept my boys successful pad users. I understand that you aren't there 24 hours a day, but maybe you can convince your son and daughter-in-law to help in the training that way.
I give my boys 2 kibble pieces for every trip to the pad, so they milk it for all it is worth.
Cleaning up the accident spots with an enzyme cleaner to remove the scent is also essential so that the dog isn't encouraged to return and mark the spot again.
Belly bands as suggested above are another option. Those take work too though to change and keep the skin clean and dry to prevent skin problems. Some dogs learn not to go while they are wearing them.