Here is a summary of the scientific paper from the Dawn Ennis piece (reference below).
tl;dr It does not explore nor provide evidence that 'kids know what gender they are from an early age'.
It is a study of 317 children aged 3-12 years identifying and living as a gender different from their sex, compared to a sibling group (n=189) and a second control group (n=316).
They ran some tests on the children, and asked the parents to fill in a questionnaire at a single point in time ie a cross sectional study.
They explored
- how strongly each participant identified with measures of gender, including preferences for toys, outfits etc,
- compared this across the 3 groups
- looked for consistency across the difference preferences asked about
- explored if there was a link between how long a child had socially transitioned, and how strong their gender preferences were
It does not explore whether 'kids know what gender they are from an early age'. It does comment that children know about sex stereotypes at an early age and supports this with a reference to another study.
'early development appears to be the time when, for example, children learn which toys or clothing or activities are stereotyped as masculine or feminine in their culture (62); the children in our study seem to have learned this information by ages 3 and 4, as even our youngest transgender participants showed clearly gendered preferences'.
I do not know how many 3 and 4 year olds were included in the study, because I cannot see in the paper, nor the supplementary appendix, a breakdown of the participants by age. We are told that the mean age was 7.62 years old, with a SD= 2.37. This is not evidence.
The discussion acknowledges
'Of course, this study is cross-sectional and captures a specific moment in participants’ gender development and our broader culture; these findings do not speak to children’s future identities or preferences.'
Similarity in transgender and cisgender children’s gender development
Selin Gülgöz, Jessica J. Glazier, Elizabeth A. Enright, Daniel J. Alonso, Lily J. Durwood, Anne A. Fast, Riley Lowe, Chonghui Ji, Jeffrey Heer, Carol Lynn Martin, Kristina R. Olson
Proceedings of the National Academy of Sciences Dec 2019, 116 (49) 24480-24485; DOI: 10.1073/pnas.1909367116