Juliet Baxter, former chairman of the PPA's national executive committee, said, 'We have had a great response. The reunion will be an informal event to mark the golden year and an opportunity to meet old friends.'
Ms Baxter said many people had got in touch in response to a letter published this summer in the Guardian, the newspaper that published the original letter in 1961 that prompted the birth of the charity.
The writer of the letter was Belle Tutaev, a young mother from London, who called for a national petition to be presented to the minister of education, demanding more nursery schools and play facilities for the under-fives.
Ms Tutaev received more than 150 letters from people wanting to establish pre-school groups and some already running them, and collected around 4,000 signatures for her petition.
The following year, 150 members attended the first AGM of the PPA. By 1966 it had 1,300 members and had opened its first office.
One aim of the PPA was to emphasise the importance of pre-school provision, and to lobby the Government to overturn Ministry of Education Circular 8/60, which prevented state nursery expansion.
'The PPA wasn't just beneficial to children and families,' said Ms Baxter. 'It grew to create a women's self-help movement. Thousands of volunteers went on to use what they had learned through their participation in the PPA to do a huge variety of things.'
In 1995 the PPA changed its name to the Pre-school Learning Alliance.