Vouchers are a little different than tickets. Vouchers can only be picked up by the person whose name is on the voucher and with photo ID.
On the tickets, you can just write the name of the child on the ticket. The signature isn't really that important for park entry. It's mostly to keep you from mixing up the tickets once the biometrics are set for that ticket. There's nothing worse than lining up at the turnstiles with mom holding all 6 tickets and not knowing which one belongs to who. If the biometrics don't work it then becomes a long process trying to determine whose ticket is whose.