Preteen years are the easiest. Young kids love you most of the time and want your loving attention all of the time. They need you to help them develop a variety of interests outside the classroom, in the arts, sports, current events, etc. Don't overdo school work at home. That's what teachers are for, and they do it best.
Teen years are the toughest. Teenagers rebel at parental guidance, but they need it, especially in relationships with others their age. Don't overemphasize grades. A B-plus student with well-rounded interests will fare better in college and in the real world than one with all A-pluses whose interests don't go beyond good grades.
Don't forget your kids in their adult years. In the 20s, they need your guidance on career choices. In the 30s, on possible career changes. In their 40s and 50s, on planning for retirement, which sneaks up on them sooner than most expect and they often are unprepared financially and emotionally.
Above all, try to enjoy it all. If you do, chances are they will too.
"That is the thankless position of the father in the family-the provider for all, and the enemy of all."
-- J. August Strindberg