my opinion now is Any child who wants to trans has to wait til they're 18
Usually actual medical procedures/hormones don't happen at super young ages. Again, the common practice is to prescribe puberty blockers until they reach an age where hormones/SRS is best.
You just described essentially the current medical practice that conservatives want to ban.
Why not let them play the sports of their choosing? Guys don't have a biological edge until puberty. A trans woman who is on puberty blockers is effectively equal in terms of athletic ability to a cis woman.