If the unthinkable happens and you already have a well-drafted Will in place, you'll have already accounted for what happens to your children and who looks after them.
What you may also like to think about is making some sort of provision to assist the nominated Guardian in covering their day-to-day living costs, schooling, clothes etc.
A Guardian Protection Plan is a cost effective way of taking out cover that will ensure that a monthly amount is paid to your Guardian to ensure that they can maintain the standard of lifestyle of your children... until they become independent.