Over 5,000 parents and concerned citizens have joined forces to take the Welsh government to court over Relationships and Sex Education lessons in schools.
The new RSE curriculum is compulsory and will commence in Welsh schools in September.
Parents are concerned that children as young as three will be taught about inappropriate topics in class, such as controversial gender ideology.
They are also angry that the Welsh government has refused to give parents "their time-honoured right" to withdraw their children from the lessons.
The parents have filed papers in the High Court in Cardiff and instructed international human rights barrister Paul Diamond to represent them.
A response is expected in early May and if a judicial review is granted, it is likely to be heard before the schools return in September.
The legal action is being led by campaign group Public Child Protection Wales (PCPW), which is crowdfunding to finance the case.
Kim Isherwood, a parent of two teenage boys aged 13-17 from Port Talbot, said most parents are oblivious to the changes coming in.
"This new curriculum is not like the stand-alone RSE lessons that pupils and parents are used to. It is embedded into every lesson across the curriculum," she said.
"The new mandatory element also means that every child aged 3-16 must take part. It cannot be avoided by anyone, and there are no rights for parents to request information on what will be taught, at what age, or to ask that their child sit out."
Lucia Thomas, a parent and accountant from Porthcawl who is backing the campaign, said the new curriculum was a breach of safeguarding and amounted to the "sexualisation of children, not the education of children".
"A friend of mine witnessed two six-year-old girls holding hands and an elderly lady said 'you two are good girls looking after each other', the two girls responded by saying 'we are lesbians and our teacher said that this was ok'," she said.
"Some teenagers, at the height of hormonal fluctuation, are told they should 'explore' whether they are transgender, simply because although a female by sex, they prefer boys' games and activities.
"We are deeply concerned that in our current culture, there is a progressive, aggressive lobby which is seeking to push onto children and young people ideologies which parents would find inappropriate."
She added, "The new Bill plays on the emotions of children and can unnecessarily confuse them. This is nothing short of a breach of safeguarding – and its being committed by our government."