When we did our will, we put in that it was our wish that X & X brought up our children but that should circumstances change, and depending on the age of the children at the time, their wishes and should be taken into account and that the trustees of our estate should consider our wishes, the childrens wishes and those of others who felt in a position to raise the children when agreeing where the children should be brought up. My husband and I both come from large families but in the main our siblings children are older than ours and in fact have grandchildren not too much younger. The children only have one surving grandparent who is fairly elderly. We then arranged that there are trustees from both sides of the family together with our solicitor to ensure that the best needs of the boys are taken into account. Since we made our will, the people we selected have split up, then gotten back together and have their own grandchild. That doesn't make them bad people or unsuitable to bring up our kids, but may make them not want to if and when the time came. At the end of the day, we are lucky in that there is no-one on either side that we would be unhappy with raising our children and I know that they would not be left with a loving home should the worst happen.
I think the best you can do is make your wishes clear to your solicitor and the other trustees and remember that it is statistically very unlikely that your plans would ever have to come into play.