If you die before your children are grown, the court may or may not appoint the person you name in your will as their guardian. Many parents believe that their provisions for a guardian are ironclad, but the nomination is only that -- a nomination based on their wishes. The court must accept the will for probate, then conduct a hearing to officially appoint and approve the selected guardian. If anyone wants to object to the appointment, he has ample opportunity.
If you and your children's other parent are divorced or were never married, and if you don't want him to have custody of them when you die, you can name someone else as their guardian in your will. It's usually very easy for a natural parent to overturn such a nomination, however. A parent has an automatic right to custody -- not guardianship -- if the other natural parent dies. The law presumes that a child's rightful place is with his parent.
If your children's other parent is unfit, this changes the situation. Your named guardian would have an increased chance of court appointment and approval if she could prove that your ex has abandoned your children, has a severe drug or alcohol problem, has a criminal record, or has a history of mental illness or child abuse. In this case, your nominated guardian can file an objection with the court and the judge will hear testimony and arguments against giving custody to your children's other parent. If your guardian can prove the case, she might receive appointment instead.
If you're married and nominate a guardian in your will, the law doesn't obligate you to choose a family member, and the court will probably uphold your selection no matter who you name. If you and your child's other parent die together in a common event, it would be much more difficult for anyone to challenge your joint selection of a guardian and have it overturned. Courts generally honor parents' wishes, but some exceptions exist. A few states, such as Massachusetts and Alaska, allow your children to object to your choice if they're 14 or older. If your children have been in the custody of someone other than your named guardian for several months, the court might overturn your selection to give guardianship to this individual. Assuming this person wanted to challenge your selection, the court might rule in her favor to avoid further upheaval in your children's lives.
If you suspect that someone may object to your selection of a guardian, you can take extra steps in addition to the nomination in your will. You can make your feelings known by adding a written explanation to your will, detailing why you made the choice you did. If it's reasonable, a court will most likely abide by your last wishes, regardless of any objections. You can do the same thing if you want to make it more difficult for your child's other natural parent to get custody. In this case, however, you would have to offer a more compelling reason than the fact that you simply prefer someone else to raise your children. You can document examples of your ex's behavior or activities that would make him an inappropriate custodian, and attach this to your will as well. As an added measure, you can write a letter to your chosen executor, stating your reasons and your wishes. The more written documentation you can provide, the better.