Your Will only speaks to the situation that exists immediately after your passing.  Your estate is closed in about a year and the assets distributed.  If you want to control how the money is used to take care of your children or delay the final distribution until your children reach a more mature age or make special provisions for the use of your estate, a trust is the answer.

You (including your spouse) are the trustee of your trust and can change or revoke it at will.  You have not given up control.  But, when the second one of you dies or becomes disabled, the person(s) you named as substituted step in, take over and carry out your wishes without probate or conservatorship. 

With a trust you appoint a trustee to administer your assets over a longer period of time.  The trustee is charged with looking out for the best interest of the beneficiaries you name and preserving the assets to accomplish whatever you have established as the goal of the trust.  You might establish the trust for the health, education, maintenance, support and welfare of your minor children delaying any large payout to them until they reach a certain age or milestone.  You might also establish the trust to take care of your elderly parents with the remainder going to your children.

Your trustee might be the same person as the guardian you name for your children.  You might feel the best guardian is not necessarily the best money manager and name different people for these roles.  Your trustee would work with the guardian to make sure the best interests of the children were met.

Trusts can be flexible and provide an opportunity for you to influence your heir's behavior long after you are gone.   Your children are much more likely to go to college or postpone frivolous spending if there are limitations on the use of their inheritance.