The union claims that around 130 of 270 children's centre staff employed by the charity have been made redundant or forced to accept new contracts on inferior terms and conditions.
According to Unison, Action for Children failed to comply with the law as the charity only consulted with the 130 members of staff working at its children's centres in Hampshire it intended to dismiss for 30 days, rather than the required 90 days.
The law states that employers contemplating making more than 100 redundancies are required to formally consult with their recognised trade union(s) for a period of at least 90 days before any dismissal notices are issued. For 20 or more staff redundancies the consultation period should be no less than 30 days.
Action for Children ended their consultation on 31 May 2012 with the first dismissal notices taking effect on 23 June.
However, Action for Children has argued that the trade union's claims are incorrect and that they only made 69 members of staff redundant, requiring them to hold a 30-day consultation with children's centre workers.
This week Unison, along with Unite, lodged a joint claim at the Southampton Employment Tribunal against Action for Children for a failure to consult as required under Section 188 of the Trade Union & Labour Relations Act (TULRCA) 1992.
Action for Children successfully bid to take over the running of 13 of Hampshire County Council's Sure Start Children's Centre 15 clusters in April after the council put them out to tender to make savings of around £20m. The charity now runs nearly 200 children's centres across the country.
As part of the agreement, Action for Children was required by Hampshire County Council to deliver savings through reorganisation of the services and redundancies.
The majority of Sure Start staff had their employment transferred from the council under TUPE. However, a loophole within TUPE allows employers to claim an economic, technical or organisational (ETO) reason for making redundancies.
According to Unison, in April this year Action for Children advised that 130 members of staff were affected by the proposed reorganisation. However as the transfer took place this figure was reduced to less than 100, thereby avoiding a 90-day consultation and saving the charity around £800,000 in wages.
Unison regional organiser Peter Terry said, 'As a result of Action for Children's failure to consult for the required 90 days, more than 100 hard-working and loyal Hampshire Sure Start workers have been dismissed much sooner than they should have been.
'It is bad enough being made redundant or having to accept a cut in pay. Being unlawfully fast-tracked through the process in this way just adds insult to injury.
'Action for Children was not forced to bid for the Hampshire County Council contract. If it can't afford to comply with both the financial restraints of Hampshire County Council's contract and the law it should not bid and instead leave the running of a public service such as Sure Start to the public sector.'
Emma Horne, Action for Children's UK director South Division said, 'The needs of our staff and the local community are Action for Children's number one priority. Making redundancies is not a decision we have taken lightly, and we understand that this is a very difficult time for all members of staff affected. These divisions were taken to ensure we could continue to meet the needs of the families we support.
'We take our legal obligations very seriously. In May this year we undertook a 30-day consultation, as required by law, with both the 69 members of staff who were being made redundant and trade union representatives. At no time did we have plans to make redundant more than 100 staff.'
The Save our Children's Centre's Hampshire campaign group has backed Unison's tribunal claim.
Co-founder Catherine Ovenden said, 'This was one of our greatest fears and what we were trying to prevent happening. Hampshire County Council promised us that there would be no loss of front-line staff. It has broken that promise by outsourcing the service to providers who are unable to comply with the financial restraints of the budget given to them.'