Oh this situation would just kill me. It kills me just to read about it. I feel sick so I know how terrible this has to be for you.

How about searching out a good but easy to understand book that covers the more basic aspects of dog ownership and give it to them as a present. Also see if you can start a friendly discussion about your dogs and why you own them and the type of joy you get by sharing your life with a dog. See if you can find out why they wanted a dog and then take it from there. The friendlier you can be, the more receptive they will be to your ideas and suggestions.
I know all this is easier said than done. Bless you for caring.