First of all, wording of the question is nonsense, as it is not the parents who "give" blood to the children.
And secondly, I don't understand some of the above answers, as the situation that the questioner might have had in mind is NOT mentioned in elders book!
In almost all countries there is the following legal situation: Doctors caring for children have to ignore parents' refusal of vital treatment. The doctors are obliged to sustain childrens life regardless of parents' objections.
WTS encourages parents and HLCs to look for doctors who show respect for parents' beliefs, that is, for doctors who do not give blood to JW children if not absolutely necessary from a medical point of view. But (I would see, fortunately), because of legal reasons, it is simply not possible for parents to hospitalize a minor AND to refuse vital treatment for that minor at the same time.
I know a few cases where JW children got blood the following way: The parents told hospital that they do not like BTs, and that they would appreciate every effort to avoid BTs. But the parents accepted the doctors right to give blood anyway; e.g. parents clearly indicated that they would not hijack their own child to bring it out of hospital if a BT was imminent. Of course, in none of those cases a JC was formed.