If you alternate weeks then it seems you have split custody 50/50. Because of this I don't understand why you should recieve child support. I can't certainly understand it if you had them more and incurred more expenses like food etc. Perhaps at your home they each have a room wheras with their dad they do not? In any case, if the split is right down the middle then he should continue to pay half of any expenses outside the home, for instance: daycare, medical co-pays and insurance, school program fees, sport registration fees, school supplies etc etc etc.
Generally the usual state-enforced child support is 20% of the income after taxes of the previous year, depending on the state and circumstances. Add 8% for each additional child (again, give or take) However, this is in cases where one parent has sole custdy and the other parent only has visitation; which is generally every other weekend and one weekday (like wednesday). You can see why the custodial parent should recieve support because of this, but in shared custody situations, there is no child support.