Praise the Lord and quit second guessing this gift of not fussing!! ;D
I actively taught my kids to sit without expression during immunizations - it really hardly ever actually HURTS, it's just that people freak out about it for no reason. Kudos to kids who don't run around screaming for every little thing!
It sounds like your children have a high pain tolerance, and a high tolerance to upset. This is a good thing. ;D
I agree with RR, don't look for trouble when there is none to be found.
My dd (now 16) had a very high pain threshold. She broke her arm and continued playing at a sunday school picnic as though nothing was wrong. I noticed (two days later) that it was slightly swollen, took her to the er, where they basically laughed at me as she was obviously not in pain. The doc was shocked to see a buckle fracture in the x-ray.
Now she is 16 and complains about every little boo boo. And while she never made a fuss over any of her early immuniations, was a huge whimp when she got her hep shots recently.