Every state is different. See what the IRS has to say about per diem - it's not taxable, it's not included in your AGI on your taxes, and it's not even reportable. From where I sit, per diem absolutely is NOT income; it's meant to offset expenses incurred as a result of being required by your job to be away from home.

Think of it this way, if your company fed you three meals a day while you were on the road at no charge to you, would your lawyer have you declare the average cost of a comparable meal in that city as income?

I am neither a CPA nor an attorney...research IRS pubs and your state requirements for child support on your own.