It must be considered from many points of view. Suppose the Watchtower is dead wrong on the issue of blood transfusion . I mean it will turn out to be wrongly understood though they thought otherwise . The mistake can be easily recompensated as we know today all small children will come back to live in the future. In death there is no elapse of time and from the eternity point of view the gaps in lost time like 100 years or 49 years are meaningless.
Also most children who continue to grow turn out to be rebellious in their later lives and many go the way of perdition. This would imply it would be better off to those children if they died in childhood as they would come back to life in the future and be saved or have a chance to be saved in the Paradise.
The blood was sacred for the Jews. To show that Jews had other foods to eat and it was not fitting to eat blood in such instances in ordinary life to show respect to God Yahweh for life and appreciation for the blood that has life in itself. The blood transfusions came into the world scene in our end-times. In extremely situation when eating other foods is not possible or woud not help to keep a person alive then using the blood to save that person could be allowed exceptionally. This view would allow the blood transfusion today to save a life. Because eating other food would not help to do just that. But with this view still we may encounter some problems.
Jesus was speaking about his blood saving life .Why he did not give some blood to drink to his disciples instead he gave them the fruit of the wine. Ah,he had to die first . If so why after his death his blood was not taken from him in small amount to keep it as it was life giving blood ? But the saving power of blood consist in fact that it had to be poured out for our sins not by drinking it. I wrote much ealier a post on the issue of blood transfusion . It is not so easy to solve . The Jews were known to rather risk their lives than to eat the sausages with blood in them when threatened with death !