Good for you for looking into rescues. There are wonderful dogs available in shelters & from rescues... many of whom their only sin is having become inconvenient.
You just have to prepare the kids for the reality of life expectancy of these big dogs, and just spend every day loving them and accepting their unconditional love in return. When the day comes for them to leave you, there is some other loving creature who is waiting for you to come adopt him/her. It's the cycle of life, and unless your lifestyle becomes prohibitave of caring for an animal the best you can, the loss of a pet should not prevent you from getting another. Sure, you can avoid the heartache of the eventual loss, but you're also denying yourself years of unconditional love and acceptance which few humans are able to give.
There are so many out there just waiting for you and your family Vince. Go for it.