The property guardian is responsible for managing the property you leave for your children honestly and in their best interests, using it to pay for normal living expenses, health and education needs. If you pick someone with integrity and common sense, your childrens property will likely be in good hands.
The property guardian must turn the property over to the minor when he or she becomes a legal adult at age 18, unless you have made other arrangements in your Will, such as a custodial account under the Uniform Transfer to Minors Act. Almost every state allows this type of custodial account, but most end by age 21. Other possibilities are a family pot Trust, which is turned over to the child at age 18, or a childs trust which is turned over at whatever age you specify in the Will. An estate attorney can discuss these options when you are preparing your Will.
If you cannot find an adult who is willing and competent to be your childs property guardian, you might want to name a financial institution to fill the role. If you make no choice and leave the selection to the court, the property guardian may be more concerned with earning a fee than with watching out for your childrens best interests.