The prediction from the International Union of Food (IUF) came just days before MPs vote on the future of the Agricultural Wages Board, which fixes the incomes of 150,000 agricultural workers and has operated since the Second World War.
In a letter to the Work and Pensions Secretary Ian Duncan Smith, the IUF general secretary Ron Oswald wrote: "Agriculture globally remains the biggest user of child labour, with 60% of all child labour taking place in agriculture."
The Agricultural Wages Board includes minimum pay rates for children of compulsory school age, and higher rates of pay for the over-16s than the national minimum wage.
Mr Oswald said: "We believe there is a strong possibility that the abolition of the AWB will make children more vulnerable to exploitation in agriculture.
"It does not make sense in the current economic climate to remove, deliberately, cash from the rural economy and certainly runs counter to the claims being made by the Prime Minister that he is concerned about eliminating world hunger, most of which takes place in rural areas, while his Government is taking measures to undermine food security for its own rural population.
"Abolishing the AWB can only be interpreted as a dismantling of agricultural workers' rights."
Julia Long, from the farm-workers' union Unite, said: "This reinforces Unite's case that the board's abolition could herald poverty wages on the land.
"It could encourage suspect employment practices and the real possibility of child labour being exploited shamelessly by ruthless bosses."
Unite is lobbying intensively in the run-up to when MPs vote on a Government amendment to the Enterprise and Regulatory Reform Bill on April 16. The amendment seeks to abolish the board, which was originally established as a wartime measure.
Today Unite members are lobbying Farming Minister David Heath in his Westcountry constituency. The Lib Dem MP for Somerton and Frome is holding a regular surgery in Frome this morning.
Unite is emphasising that Mr Heath had previously expressed his support for the retention of the AWB.
Mrs Long said: "David Heath is being lobbied as he is responsible for driving through the proposed abolition of the AWB, having previously supported it."
The Government maintains the AWB has been superfluous since the advent of the minimum legal wage.
