Retraining Golden Retrievers

Question
We have 2 beautiful Goldens, ages 6 & 8.  They are very well behaved and one of them is practically human.  They are trained to relieve themselves in the backyard, but not in a particular spot. The only problem is that we have 3 children now and we cannot let the children outside to play until we clean up all the poop first, because unfortunately the girls (that's what we call the goldens) were not trained to go in just one area of the yard.  Therefore, they have free reign of the entire backyard as their bathroom.  How do we re-train 6 & 8 yr old goldens to go in one spot, when their environment hasn't changed at all?  They are living in the same house they grew up in, we are simply just trying to correctly train them to go in one spot rather than anywhere in the backyard, obviously for sanitary purposes for our children, guests, etc. We do not want to confuse them and they are not doing anything wrong, as they are doing what they were trained to do. However we have realized that they should have been trained to go in one spot, rather than the entire backyard. Any suggestions would be greatly appreciated.

Answer
I don't know if it can be done or not.  I would try what I do with young puppies.  

Choose a command and spot you want it to use. The less accessible to strays, the less chance of serious disease. If it is a female, choosing a non grassy spot will avoid brown spots later. When you bring it home, take it to the spot and give it the command in a firm, but friendly voice. Keep repeating the command and let the puppy sniff around. If it does anything, praise it. Really let it know what a good dog it is and how much you love it, and maybe a treat. Note, being out there not only means you can praise it, but it also keeps it from being snatched by a hawk. If it doesn't go, take it inside and give it a drink and any meals scheduled. A young puppy will need to go out immediately afterward. Go to the spot and follow the above routine. Praising it if it goes is extremely important. If it doesn't go, take it back inside and put it in its crate and try again soon. Do not let it loose in the house until it does go.

Activity stimulates elimination.  You may need to walk them around or even play fetch or something in the area you want them to use.