I have been looking at various websites and they all agree that benefits are not used to pay into a bankruptcy. While the child maintenance would be looked at as a part of the household income, it would not be used to pay into a bankruptcy.
All income including all benefits are used to work out how much your surplus is. But the payment is only taken from the earned part. This changed about 18 months ago befre which child benefit was totally excluded.
The Insolvency service has provided this explanation of how things are worked out