Parents whose kids have their own passports should apply for a visa for their child separately. Furthermore, they must accompany their child on a trip to Vietnam. If the parents are not around, a guardian or an authorized adult who has the permission to bring the child to Vietnam should accompany the child when passing through airport immigration or passport control.
The child will be granted his or her own Vietnam visa and you need to bring this with you on your trip. If the child is below 14 years old and he or she shares a passport with the parent or guardian, then he or she will also share the Vietnam visa that was granted to the parent or guardian. It should be declared on the visa that the adult is sharing it with the child.
So if your child is less than 14 years old and he or she doesn’t have any passport yet, you must indicate on your visa application that you also need a visa for your child who will be with you on the trip.