Plaid Cymru wants youngest taught mainly in Welsh
Schools in Wales should teach three to seven year olds mainly through the medium of Welsh within 20 years, Plaid Cymru has said.
Half of English-medium schools should be doing it within 10 years but Welsh-speaking areas would be the priority for its new Foundation Phase policy.
Education spokesman Simon Thomas said bilingualism was "a valuable skill".
The National Union of Teachers said there was "merit" in the plan but asked if sufficient teachers could be found.
Plaid Cymru was launching its language strategies ahead of May's general election at an event in Newcastle Emlyn, Carmarthenshire, on Friday.
"Accessing services and operating in the workplace in their language of choice is a basic right for all the people of Wales," said Mr Thomas.
"Nowhere is this right more important than in our schools and playgrounds where the foundations of children's confidence to converse in Welsh is formed.
"We know that bilingualism is a valuable skill and benefits children's cognitive development."
Owen Hathway, Wales policy officer for the National Union of Teachers, said there was "merit" in the plan but added that "finding enough teachers capable, and wishing, to teach through the medium of Welsh can be difficult".
Rex Phillips, national official in Wales for the NASUWT, asked whether Plaid Cymru had considered "parental choice and the views of pupils, teachers and the wider education workforce before making this announcement".