A US minor's passport still has 2 years remaining, however, is it possible to preemptively extend it, anticipating a foreign relocation?
You may apply for a new passport at any time. Link The ability to obtain a new passport does not change whether a minor or not. But you may not renew a minor's passport.
All passport applicants under 16 must apply in person using Form DS-11. You can not use Form DS-82 to renew your passport.