The foundations of good mental health and good physical health are basically the same. Adequate sleep, a healthy diet and consistent physical activity keep kids (and adults) feeling mentally and physically at their best.
Beyond those basics, good mental health is in many ways about relationships: learning how to interact with and relate to others in constructive, mutually beneficial ways. So be sure to devote time to your relationship with your child. Make time for fun and play, and try not to overschedule. If possible, leave time in the week that’s free and open, when your kids can spend time with you and talk or play.
Perhaps most of all, focus on the positive. Instead of devoting attention to the challenges of your relationship, focus on what’s good, what your kids are doing right. Your kids want your approval. Often, promoting the positive will take care of the negative on its own.