Yes, DHS had made it mandatory for a child of any age to appear in person and give their fingerprints for their application. It was done for i539 based applications like H4, L2, etc but now they may add it for all types of US visa extensions within the US. DHS's intent to collect biometrics at any age is to match the records created for children to their adult records later and help combat child trafficking, smuggling, and labor exploitation by facilitating identity verification.