If they're under the age of 19 (or 24, if a full time student) you don't need to provide the amount of support, just that you were the custodial parent and that they lived with you more than 1/2 of the year. That can be done with school records, medical records, affidavits from neighbors knowledgeable with the situation, etc.
None of this needs to be sent in with the return.
