Blower wants the government to develop a play-based curriculum to help young children to develop confidence and independence.
She said the current focus on testing ignores young children’s social and emotional needs, setting them up to fail within education at an early age.
Blower’s remarks came at the union’s annual conference which has been debating the government’s plans to introduce in 2016 compulsory tests in numeracy and literacy for children as young as four.
She said: “The emphasis on formal learning and assessment is putting undue pressure on our youngest pupils.
“Children and young people do not develop at the same rate and this approach takes no account of either summer-born children or those with special educational needs.
“A play-based curriculum is what is needed – not unnecessary tests, which children in their earliest years of education might ‘fail’, giving them a negative message in the earliest years of schooling.
“We need to be inspiring young children, not making them afraid or bored through a task-orientated curriculum which simply stultifies the learning process.
“Being confident, independent and curious is as important as cognitive academic skills and must be defined in the light of children’s diverse abilities.”
Blower went on to back the Too Much, Too Soon campaign, launched in September last year, to petition politicians to introduce a “developmentally appropriate” policy for the early years with a focus on play.
She said: “Many European countries do not begin formal education until the age of seven.
“The difference of course is that, particularly in Scandinavian countries, there is a high-quality pre-school provision using a play-based curriculum as an entitlement, which does prepare children well for more formal learning later.
“The educational outcomes for these countries are in no way negatively impacted – in fact, they are often held up as an example of excellence by Michael Gove.”
Blower's comments have been welcomed by Liz Bayram, chief executive of the Professional Association for Childcare and Early Years, who said a play-based curriculum will better prepare children for life.
She said: "The evidence shows that high-quality childcare delivered through a play-based approach to learning is essential to helping children develop the social, emotional and physical skills they need to thrive and is one of the most effective ways to lift children out of disadvantage.
"A child's confidence, independence and willingness to learn is more important than being able to recognise letters, sit still and focus on a task."
A spokeswoman for the Department for Education defended the plans to introduce testing at an earlier age.
She said: "We are working with teachers to raise the bar to improve standards in primary schools and introduce a proper measure of progress from when children start school to age 11.
"Under our proposals to improve primary accountability, schools will be held to account either for ensuring all children make sufficient progress from reception to the end of primary school, or for ensuring at least 85 per cent achieve the expected level in reading, writing and maths."