Toilet training lab

Question
Our 4 month old labrador is fully house trained but would like to persuade him to use a small area of the garden to use as a toilet rather than litter the entire garden with poos - not good with 5 samll children as we always seem to miss one! ... and the children don't!

Any clues?

Answer
It may be a little late to start.  It will take a lot of work now.  This is what I do with a younger puppy.

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.