How do you determine who is going to pay for what expenses if the kids spend equal or substantial time with each parent?

That is an excellent question that has come up more and more frequently in recent years and decades since the "every other weekend" model stopped being the only parenting schedule we knew how to use. Before, one parent spent the vast majority of the time with the kids, so that parent always purchased anything that wasn't a day-to-day expense such as food or utilities. Now, however, kids spend time during the school week at the non-custodial parent's house, far more than four nights a month. The "let me write a child support check and then you take everything out of that" approach no longer works, yet it is still used in most cases because the lawyers and the parties are often not thinking about the long-term effects of vagueness. It is best to delineate in your divorce agreement who will pay what share of the following types of expenses: clothing, school fees, extracurricular fees, sports costs, photographs, travel, field trip fees, lessons, tutoring, and the like.