Extend the right to ask for flexible working to all parents, not just those with children under five. AND strengthen it, don't just make it being able to ask the question, for heaven's sake.
Actually extending it to all employees might break down the resentment some non-parents feel towards parents (that old 'maternity leave is a holiday' attitude).
I know you asked for one thing, but also give the Inland Revenue the task of collecting and assessing maintenance, seeing as they are good at getting money out of people.
AND (I know, I'm going on, pick the one you like best) do something about housing. Build more council houses/social housing. Price of houses/fact that rental properties are generally only available as shorthold, six month tenancies makes life very hard for families.
And do something for SAHMs. If people wish to SAH, they shouldn't be penalised. Daft where govt. is happy to pay towards someone else caring for my child, but not support me to do it (have never been an SAHM so no axe to grind here).
AND sort out flipping tax credits - confusing mess that is bloody unfair to people who are reassessed and get money clawed back. Does not help poor families. He has to make it work in practice, not just in theory.