My son’s cut down putter seems to work well and it looks to be flat enough when he addresses the ball. Kid is 47” tall and putter is 25.5” finished.
We never had to adjust lie on putters. Irons, yes. Check with a pro to see if her setup if off (e.g., hands too low). If your child prefers straight arms down (e.g., chambeau), you may have to go to even a shorter putter or have them choke down.