First thing I wanted to ask...Did you fill out a medical release and have it notorized? Before you leave make sure your children's care taker has one in case of an emergency.
I HATE this part of parenting but it is a neccessary one. The key is to review your will often as time changes people and situation. As far a choosing a gaurdian I think your best bet is to go through the potential person(s)list with your spouse and pick the one you find best suited. I would start with immediate family, is there a brother, sister brother/sister-in-law that could provide care. I would think that they would have a vested interest in you so that interest and care would continue to your child. If they aren't able perhaps a family friend that you have come to love and trust that fits the appropriate age range and has the desire to raise children.
Secondly, don't forget to approach this person and ask them if they are willing and able. Perhaps you could set up a trust fund so they have some financial help if needed.
Best of luck, this is never fun but one of those things that has to be done!