I will reword your argument into a syllogism. Tell me if I am understanding you correct
premise 1- If something is extremely painful to infants, it should not be done to infants.
premise 2- circumcision is extremely painful
conclusion- circumcision should not be performed on infants
If this is your argument than you have done a great job supporting premise 2, but have done noting to support premise one.
I don't think you can successfully defend premise 1 anyway. clearly some painful things should be done to infants, for example if they are burned in a fire and need skin grafts. If I had to steelman your argument, I would change it to the following.
premise 1- if a medical procedure is painful to infants and unnecessary for the infants well being, it shouldn't be done to the infant.
premise 2- circumcision is painful and unneccessary
conclusion- Circumcision should not be done
The second version of the argument is known as a "strong argument". This means that it isn't always true but mostly is and it appears cogent at first glance, assuming you can support the procedure being unnecessary most of the time.
Am I properly steel manning your argument?