It also says a pilot scheme of the expansion should be given adequate time to see whether it works, and that the government should work more closely with the Church to deliver more childcare.
The Forum was speaking after David Cameron announced his government would be bringing forward a previous pledge to double childcare for three and four-year-olds in England.
It was previously meant to come in September 2017, but a pilot scheme for some children is now coming in September 2016 instead.
At the moment, every three and four-year-old in England, in households earning less than £150,000 a year, is entitled to 15 hours of free childcare a week for 38 weeks of the year. This will be doubled to 30 hours a week for children in the pilot.
It's set to cost taxpayers £350m a year, however the Employment Secretary Priti Patel says the final exact costs haven't been worked out yet.
However the Pre-School Learning Alliance - one of the biggest early years learning organisations in the country - has said that the amount of funding the government currently gives to nurseries for the free 15 hours a week isn't enough, and this funding shortfall will only get worse with the expansion of childcare.
Specifically, the Alliance has said that while the government gives nurseries £3.88 per child per hour to provide free childcare, the true cost of this care is actually £4.53 - about 20% more than what they're being given.
The government has acknowledged funding rates given to nurseries need to increase.
The Alliance also thinks free childcare should be more means-tested, and that nurseries will not have enough space to deal with the extra care it has to provide.
Keith White, from the Christian Childcare Forum, told Premier's News Hour: "I'm pleased to hear that the government's taking this seriously.
"I understand that it's trials that are being brought forward... it's going to be looked at, explored, and the whole point... is to see how this works.
"I think there's a missing element here. What I'd really like to see would be churches and voluntary nurseries... getting their heads together to talk about space, and also to talk about figures, and talk about other options.
"It was actually this Prime Minister who introduced the notion of a 'Big Society', and this seems to something where if ever 'Big Society' meant anything, we now need to make sure that his minister Priti Patel is exploring how this can work without simply more bureaucracy and simply assuming state funding, but how it can work where we can all play a part."
Listen to Premier's Antony Bushfield speaking to Keith White on the News Hour: