My DD divorced her OH when her son was two and a half, in a way it was harder as he was too young to understand the reason, which now at seven he does. However the best advice I can offer is to make sure both you and they NEVER say any thing against the each other. My DD and her ex have been very hot on this, and it is all very civilised and I think this helps a lot. I always say to my GS when he goes off for the weekend with his Dad "have a lovely time " and ask him if he had a good time, and then listen to his stories when he gets back and comment positively. It is terribly hard when he cries for his Daddy, and we are very aware that he may well choose to go and live with his dad when he gets older, which is distressing. I just make sure I tell him how much we love him and when he cries agree that it is hard.
The other thing my DD and her ex always does is to be very flexible about the times he is at his Dad's house to suit every one so that it fits in with my GS's clubs, friends, parties, holidays etc. This makes it easier for my GS I feel as it is all more relaxed.
They actually live with us so I can give him plenty of love and be there for him and also he has my OH as a male role model, which is helpful too.
I feel for you plinkplonk, especially when the papers are full of all the ideas about how divorce affects children. I am not convinced that it has to affect them badly if it is handled well xx