I don't know about 3 kids.. but at Conrad HK, they never charged my kid for breakfast. They asked for my room number/last name the first day and seat us. After that, each morning they knew us and didn't ask for room number again.
At Conrad Seoul, they would randomly charge my kid for breakfast. Some day it would be free. Some day they would charge him as adult. Some day, they charge kid price. It was really strange.
All the Conrads I have been to don't use voucher. They just ask for room number/last name and that's it.