Children as young as four are being asked to choose the gender "they most identify with" on a primary school form.
Brighton and Hove Council sent a letter to thousands of parents this week, asking them to support their child's gender choice before they start school for the first time in September.
It also allows them to leave the form blank if their child had "another gender identity", according to The Sun.
One unnamed mother told The Sun: “They are not adults - let them enjoy the innocence and creativity of their childhood.”
Conservative MP Andrew Bridgen said the letter was "utterly ridiculous".
“Schools should be teaching kids to read and write, not prompting them to consider gender swaps,” he told the newspaper.
The council has defended the letter.
Councillor Emma Daniel, head of Brighton's equalities committee, told BBC Radio 4 that the council were "not expecting parents to have any sort of conversation" about gender and identity with their four-year-olds.
She also confirmed to The Sun that it would be reviewing "concerns" over the wording following the negative response.
"We have inserted the additional text about gender identity in response to calls from families, young people and schools to show an inclusive approach.
"There are increasing numbers of children and young people nationally identifying as trans."
She added: "We will review this to see whether we can make it clearer that we consider discussions around gender identity to be an option for parents rather than an obligation.
ITV News has contacted Brighton and Hove Council for further comment.