There are now about three or four studies where it has been identified that from the age of 6 male children have a physical advantage over female children.
Danish study on VO2 max and LBM in children from age 6
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1725036/pdf/v039p00725.pdf
Maximum oxygen uptake and objectively measured physical activity in Danish children 6-7 years of age: the Copenhagen school child intervention study
Eigberg, Hasselstrom, Gronfeldt, Friberg, Svensson, Anderson
October 2005
Objectives: To provide normative data on maximum oxygen uptake (Vo(2)max) and physical activity in children 6-7 years of age and analyse the association between these variables.
Methods: Vo(2)max was measured in 366 boys (mean (SD) 6.8 (0.4) years of age) and 332 girls (6.7 (0.4) years of age) from preschool classes in two suburban communities in Copenhagen, during a progressive treadmill exercise. Habitual physical activity was measured with accelerometers.
Results: Boys had higher Vo(2)max both in absolute values (1.19 (0.18) v 1.06 (0.16) litres/min (+11%), p<0.001) and relative to body weight (48.5 (6.0) v 44.8 (5.6) ml/kg/min (+8%); p<0.001) than girls. The difference in Vo(2)max between boys and girls decreased to +2% when expressed relative to lean body mass (LBM). Absolute Vo(2)max was related to LBM, body mass, and stature (all p<0.001). Boys were more physically active than girls (mean counts +9.4%, p<0.001), and even when boys and girls with the same Vo(2)max were compared, boys were more active. The difference in physical activity between the sexes was higher when sustained activity of higher intensity was compared.
Conclusions: Vo(2)max is higher in boys than girls (+11%), even when related to body mass (+8%) and LBM (+2%). Most of the difference in Vo(2)max relative to body mass was explained by the larger percentage body fat in girls. When boys and girls with the same Vo(2)max were compared, boys engaged in more minutes of exercise of at least moderate intensity.
Australian children
https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=021cccdaed57d120bb05bac71c05ee82b0c5b315
Greek children ( I have no access to this other than the publicly available)
https://www.tandfonline.com/doi/full/10.1080/17461391.2015.1088577?needAccess=true
Here is a new study released on sex differences between male and female children under 11 years old.
Sex-based differences in track running distances of 100, 200, 400, 800, and 1500m in the 8 and under and 9–10-year-old age groups
Gregory A Brown, Brandon S Shaw, Ina Shaw
5th February 2024
https://onlinelibrary.wiley.com/doi/10.1002/ejsc.12075
"In conclusion, although some have stated that sex-based differences in athletic performance do not arise until puberty, the present data indicate that in the 8 and under and 9–10-year-old age groups males run faster than females in distances of 100, 200, 400, 800, and 1500m. While some females in these age groups are faster than some males, the average male finalists are faster than the average female finalists, and the fastest males are faster than the fastest females. As running is a key component of many sports, these sex-based differences between prepubertal males and females should be considered when sport governing bodies and policy makers consider the issue of sex-based sporting categories"
I would suggest that organisations who run the children together are fine to some extent if they reward each sex. However, I think that some studies would need to be done as to whether racing with male children deters female children from competing. It may prevent a female child from entering in the first place, even knowing the results are to be split.