The custodial parent in your particular case will be the parent receiving the child support. If custody is split 50/50 then it is the parent with the lower income. Child support is paid to you to feed, clothe, and house your children. Any other expenses for your children are add-on expenses and are dealt with differently than child support.
Child support payments are determined by a basic calculator, using both parties’ incomes, to determine the basic child support obligation of the non-custodial parent. The add-on expenses are based on the percentage of your income against the total income of the parties. The add-on expenses would include unreimbursed medical, medical insurance, extracurricular activities, education, camp, etc. These expenses would be paid by your pro rata share.
If you choose to negotiate the terms of your divorce with your spouse, you can be creative in the way you would like to split these expenses. If a judge is making the decision for you, it will simply be based off the calculators’ results based on your income.