Hi tg and welcome to the forum
Yes your child maintenance is included as income, as your household expenditure will also include provision for the children's expenditure in terms of food, clothing and educational expenses.
I don't think your partner's argument would have any substance, and he does not need to know about your IVA in any case. Child maintenance payments are set as a percentage of the father's income depending upon the number of children, and have no relevance to what the money is actually spent on.
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
For further details contact me at
http://www.melaniegiles.com and view my IVA blog at:
http://melaniegiles.blogs.iva.co.uk