Maintenance is money to assist you in looking after the children -- it is not theirs. As such it will be counted as income. The expenses of bringing up the children are catered for in your agreed allowances .. you can't have that money allowed for twice.
Is the father paying under a court / DWP order ? If not, and he just "bungs you a few quid every now and again" (i.e. an informal arrangement), could he, instead, buy the kids some clothes, toys, do a food shop himself and give this to them ?
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk