In a tonsillectomy of a child, what are the chances of needing a blood tranfusion during surgery?

Very rare: In todays times it is very unusual to require blood transfusions for a tonsillectomy. The procedure is very different now and is an out patient procedure.

Nearly none: It is vanishingly rare to receive a transfusion during the tonsillectomy. One needs to be careful, however, during the aftercare period. This is when bleeding can be dangerous, especially for small children. Talk to the surgeon about their perosnal rate of bleeding and what instructions s/he would give you should that occur.