There are two main reasons children don't usually testify in custody hearings. For one, testifying can negatively affect the children, especially when they have to say negative things about a parent (whom they love). Also, custody determination is up to the judge and the law, and not the kid. Still, your kid may have something to contribute to their custody case. Here are three cases in which it's advisable and reasonable for a kid to testify:
If you think your spouse has a drinking problem and you can't get them to reach out for the help they need, you may be considering a divorce. If you think that you want to try to get them help, and that you want to see if things could work out if they stopped drinking, you want to talk with a lawyer to see what your options are.
You may want to take some space from your spouse to let them know you are serious about their drinking problem and that it has to stop, and that you have met with a legal expert about possibly separating.