There is no easy time to tell them. Wait too long, and it will be more of a shock on them. Some even have become resentful at that.
They say that kids are resilient...yes they are, but they will always carry these scars with them. And they will be afraid that somehow its their fault. Be strong and there for them.
There are moments in this life when you are so confident in the rightness of your actions, that not even for a second do you consider the option that you might be wrong.