If both parents die before their children have reached adulthood (18) legal guardians will be appointed to care for and deal with financial and legal matters concerning the children.
If no guardian is appointed in either Will the court will appoint guardians.
You should therefore appoint guardians in your Will. Ideally you should also appoint alternate guardians, in case the named guardians have died.
You can specify your choice of legal guardians in your Will. How to choose who you appoint as a guardian can be a difficult task. Take your time choosing and ask yourself who you would feel most comfortable with caring for your child. Make sure to speak openly to your family about this and get the consent of the guardian(s) you want to appoint.
You might want to name a set of two guardians, one in charge of the day to day care of your children and one to deal with all financial matters.