When I enter Sarah Teather's office, the minister for children and families is celebrating a victory over the forces of bureaucracy. The windows in her top-floor office, overlooking Great Smith Street in Westminster, open wide enough that a careless person might feasibly plummet to certain doom below. So they are kept locked. Even ministers do not have the key. But on this bright spring afternoon, Teather has, after a nine-month campaign, secured access to fresh air.

This triumph aside, the atmosphere around the Liberal Democrats' role in government is gloomy. The weekend before our interview, delegates at the party's spring conference have both supported and opposed government plans to reform the NHS in rival motions. That does not, I suggest, indicate a happy party. "It is clear that people are still uneasy," Teather concedes. "We need to work harder to make sure that they understand exactly what's been achieved."

We're supposed to be talking about education, not health, but I'm intrigued by parallels between the two agendas. The academies and free schools programme is also predicated on the belief that competition from new providers will drive up the quality of service. Lib Dem grassroots hostility to this mechanism in the NHS is well-advertised; I pick up similar suspicion regarding schools. Has Teather detected the same?

"At a Lib Dem conference you'll find people saying 'well, that doesn't feel very Lib Dem' - of course it doesn't, this isn't a Lib Dem government, it's a coalition." Does she personally believe that competition from the private sector is the best way to drive up standards in public services? "I don't have any ideological objections to the use of the private sector. The Liberal Democrats have never had any ideological objections to the use of the private sector, that's the same in health and in education." What about the prospect of companies making a profit from running schools? "That's different. That was one of the key dividing lines that Nick Clegg made clear. Free schools will not be making a profit during the life of this coalition."

Teather's main ministerial focus is on what happens to children before they reach school and one of the policies that Lib Dems are keen to promote as one of their contributions to government is a substantial increase in free nursery places - providing 15 hours of care per week to 260,000 more two, three and four-year-olds from families on low incomes. I wonder if this message has been obscured by cuts elsewhere, to Sure Start children's centres, for example. Teather insists reports of Sure Start butchery are exaggerated. "There's a lot of talk about local authorities scrapping children's centres but the evidence doesn't stack up. We still have 3,500 across the whole of England." Departmental figures, last collated in September, claim 124 centres have closed so far.

Meanwhile, proposals will shortly be announced to get parents more involved in running children's centres, borrowing perhaps from the model of school governing boards. The idea is meant to form part of a theme of parent-driven accountability. The same motive is behind new rules, due in September, that will force schools to publish how they are spending their "pupil premium" - a signature Lib Dem policy that allocates extra money for children entitled to free school meals. The idea is that greater transparency will put pressure on schools to narrow the gap in attainment between children from poorest backgrounds and the rest. "Historically they under-perform on their potential and at the moment schools aren't adequately getting to grips with those issues," says Teather. "A school can't claim to be performing well if it's leaving behind some of those children at the bottom."

The obvious danger is that any ambition to help children from poor backgrounds is sabotaged by welfare and tax credit cuts, which are due to make life substantially harder for low income families. Teather was a critic of government plans for a cap on the level of benefit any household can receive. In February, she missed a crucial parliamentary vote on the measure - a highly irregular ministerial abstention. It prompted calls from some Tories that she resign. So does she now support the policy? "I am on record as having concerns about the benefits cap, but I am also pleased to see the changes that were brought in on the back of those concerns, my job is sometimes to take points and suggestions about things that other departments are doing that affect children and families. That's the job that the prime minister asked me to do and I take it seriously."