If the English standard of your child is not that good, then the earlier you go to school in the US, the better.
It will be difficult to get into a good school in the US if the English standard is not there.
Some boarding schools do not take students after 9th grade, especially the smaller ones, so your choice is limited.
US colleges would like to see school report for at least 3 years. Which means that your child needs to start getting good grades as soon as he/she is there. If you go in G10, there’s little preparation period.
Going early will allow your child to spend some time getting used to the curriculum and the way of teaching.