In an adenoidectomy of a child what are the possibilities of needing a blood tranfusion during surgery?

Minimal: If you are seeing a reputable ENT physician, are getting care in a reputable hospital or surgery center ? Using a reputable anesthesiologist, the risk is minimal. Proper screening and examination and appropriate testing for bleeding disorders ? A thorough review of medications being taken should be done. Ask questions and seek a second opinion as needed or if unsure. Ask how many have occured?

Near to zero: Although it is theoretically possible in the case of unsuspected bleeding disorders or really botched surgery, in my 26 years of ENT i've never heard of a case.