There are no hard and fast rules for what support is supposed to cover. The statute states “payments ordered for the support of a minor child shall be in such amount as to meet the reasonable needs of the child for health, education, and maintenance, having due regard to the estates, earnings, conditions, accustomed standard of living of the child and the parties, the child care and homemaker contributions of each party, and other facts of the particular case.” If you are on Worksheet A, the guideline amount is supposed to cover all expenses except medical over $250 per year so you have no obligation to provide additional support. Worksheet B should be used when there is a true sharing of expenses between the parties.
The idea is that you have to provide a certain level of support for the children while in your care (food, clothing, shelter), but I also try to indicate in the agreement how other things will be split (like medical, dental, extracurricular activities, etc.). If your agreement doesn’t state with particularity what additional items will be split between the parties, you have no legal obligation to pay them.
If you feel compelled to pay for these items outside of the order, a good way to determine what to pay is by looking at how much overall support you are paying in comparison to the other parent and the disparity in your income. The more communication you can have about these extra expenses the better. For instance, both parents will need to provide clothing for the children while at his or her house, but each child really only needs one winter jacket and one pair of sneakers so splitting the cost of these would seem fair. Similarly, it doesn’t seem fair for one parent to determine what extracurricular activities the children will participate in (especially if they are expensive hobbies/sports) and simply expect the other parent to pay half.