The Stanford Children's Hospital [1] will provide surgeries to both minors and adults. It doesn't specify how young the children can be.
The University of Illinois [2] will do surgery on people under 18, but doesn't specify how young the children can be.
The University of Rochester [3] will do some surgeries for people under 18. It doesn't specify the age, but it is under the "Adolescent Medicine" section.
Seattle Children's Hospital [4] will provide referrals for gender-affirming surgery to children as young as 9 so long as they have started puberty.
[1] https://www.stanfordchildrens.org/en/services/gender/what-to...
[2] https://hospital.uillinois.edu/primary-and-specialty-care/su...
[3] https://www.urmc.rochester.edu/childrens-hospital/adolescent...