Sir Paul Ennals, chairman of the Children's Workforce Development Council, has been advising the Treasury and Department for Education (DfE) on how to preserve services during the current austerity.
He said he had never seen such "ashen faces" among officials in the Government meetings he had been attending.
Sir Paul, also chief executive of the umbrella charity the National Children's Bureau (NCB), was speaking last weekend at a gloomy annual conference of the Aspect trade union.
Representing education professionals, many working in local government, the union offered members seminars on coping with redundancy and how to protect their pensions.
Sir Paul estimated the DfE would have to find cuts of around 15 per cent before the autumn comprehensive spending review.
"With school budgets protected, we are talking about anything up to 45, maybe 50, per cent missing from non-school budgets," he said.
"I don't yet believe that the Government will find themselves able to follow that through whilst still seeking to deliver the policies that they want to do.
"But we have got to realise that these three months in budgetary terms are going to be the toughest and the dirtiest inside Government that it has ever been.
"Part of my role in NCB is seeking to advise the Department and the Treasury on some of the most effective ways of following through that process, and some of the ashen faces that I saw yesterday at some of the meetings that I was in were of a type that I hadn't previously seen before."
John Chowcat, Aspect general secretary, said his members were facing "extensive service restructuring and job insecurity".
He said Government was encouraging schools to leave local authorities and become academies "without careful piloting or sufficient supporting research evidence".
He noted that Lord Hill, the Conservative education minister, had told Parliament about the future role of local authorities in education, saying "I recognise that the coalition Government have not yet come up with a satisfactory answer on what it should be".
Mr Chowcat is concerned that ministers could be about to let market forces rip without ensuring minimum standards were met and said they were risking "expensive failures". But he was confident that "better times" would return.
Sir Paul said there were "reasons to be cheerful" and that children's services would quickly replenish themselves like eucalyptus trees after a forest fire.
In the meantime the answer was to try and get broader children's services delivered through schools.
"We have to strengthen the ability of the frontline (school) universal services to meet more of the needs currently being met in other places," Sir Paul said. "There is a sort of benefit in this - of empowering the front line.
"There will be budget reductions that we as a sector need to stand up and shout about," he said. "But if we waste our collateral by complaining about every single budget reduction along the way, we will not be listened to when that key moment comes, when they are making a grievously wrong strategic error in a decision that they're proposing."