Kudos to the Wall Street Journal for letting parents know the importance of setting up trusts for their children upon their death. Generally, if you don’t have a Will, or have one that leaves everything to your minor children outright, then upon that child reaching the age of 18 (or 21 depending on the state), the child will receive all of their money outright, without any supervision. Also, before they reach the age of majority, a Guardianship (which can be expensive and burdensome) might be required.
One thing that I think the article does not make clear is that you do not have to set up a separate trust while you are alive. A revocable trust is proper for some people, and others transfer money or property to an irrevocable trust for their children’s benefit for estate planning or other purposes. But for most people, the trust for your children is part of your will. It does not come into effect until after you die (or upon the second death of you and your spouse).
For more, see Deciding if Your Kid is Trust-Worthy.