Are donor babies ethical?

Let’s give you a scenario, you have a very very sick child who will require transplants throughout their life. They are your only child and the doctor gives you the possibility of having another a child who would be the same match for bone marrow, blood, etc and could save your existing child and give them a good shot at life. So would you, as a parent who has exhausted other options, have a donor child to keep that existing child alive? Additionally, is it ethical to have a child purely to save the life of your existing child? To have their whole life centred around saving the life of their sibling? For that to be the only reason why they existed in the first time? Would you have one yourself?