You can be either in the season of kids or in the season of empty nesting. Both seasons are dully important and illustrate the need for a will.
Let's start with the seasons of kids. In this case, a will is a starting place and better than nothing at all. I have always thought of a will as a wish list. A will goes through probate and the courts are charged with the responsibility of determining whether your wishes are to come true. Setting up a trust will keep those assets and your children out of the validation of the court system.
With a trust in place, you can specifically dictate how you want everything distributed as well as your kids taken care of. A few big decisions include who will be their guardians and the restrictions you put on the money that you leave. After all, we don't want 16 year Billy to have the freedom to spend 100,000 on a Porsche. Of course, you will also have to select a trustee for the process. I always advise to pick two different people - one to be trustee and one to be guardian. I think it is to much power to give one person. Checks and balances are a good thing.
Additional Step - Both husband and wife should make sure that they have plenty of life insurance in place. Go through the process ahead of time of creating a plan that takes care of the surviving family in case of an unexpected death. It is so very inexpensive to fund term life and get your family covered.
Now what about if you have adult kids? I would give the same advice. A will is better than nothing. Having a detailed trust in place is much better and helps to prevent two challenges you leave behind. First, the kids are informed and are not searching for answers.
More times than not, adult kids are left in the dark when it comes to their parent's finances. This way the parents wishes are known and there is no questions...which leads to the most important point. Second, you reduce the probability of fighting. Money has a way of bringing out the worst in people and there is nothing like the subject of money to create a fight.
Additional Step - For the parents, it is also important to have everything clearly outlined in a folder of some type so that the kids know where everything is and are not left wondering are searching for important papers.