I dont think it even matters when it comes to godparents. I just thought it was more of a honour thing than a legal thing. My godparents haven't contacted me in over 20 years since i was christened (I'm 22 and dont even know what they look like let alone their names) and I wouldn't of 'go to them' if the worse case senior ever happened - even when I was say 6. Grandparents where the first choice. So just dont contact them and your kids will soon forget them - they cant take your kids away if anything happens as they have no legal standing!
As Mintox said, being a god-parent doesn't bestow any legal obligations at all, so you don't need to do anything to formally remove their status as god-parent. Traditionally, a god-parent is there to offer christian guidance etc. I suppose you could have your children baptised again or blessed and use this as an opportunity to put in place substitute god-parents?