1.   A child may be wealthier than his siblings
2.   A child may be a poor money manager
3.  A child may have a failed or poor marriage
4.  A child may have a disability
Generally, instead of disinheriting a child, or leaving one less money, a client can incorporate a trust into her estate planning.  A trust can hold money for the child who can not manage money, who faces an ex-spouse, or who has a disability and can not own money if she were to qualify for government assistance.  Similarly, a generation skipping trust can own funds for a wealthy child so the child is not taxed on the inherited funds at his death.

Trusts are like beer, chocolate, and coffee in that there is not anything they can not do.