I explained we were going to the doctor to get her some medicine which would keep her healthy. She understood that concept because we got our flu jab/spray together the year before and she saw me get the jab (even when she got the spray). I explained where they would do it, in her arm, and that it would be a little pinch in each arm, but then it would feel better. She asked a few questions about it and I was honest with her that it might hurt and it might bleed a little, but it would keep her healthy and that was fine. She handled it all very stoically. I think it's good to be open about it, but not make a big deal of it either. It will hurt a little, but it is good for them and you'll be there for him. It really wasn't a big deal and actually much less traumatic than I expected!