How long after para-umbilical hernia op can I lift a two year old?

About 2 weeks. If your two-year old is about 20-25 pounds, then, depending on the size of your hernia and if it is repaired laparoscopically (with tiny incisions and a video camera) or open (single larger incision), then you should be ok to lift your child in about 2 weeks.
Ask your surgeon. Most surgeons recommend avoiding heavy lifting or heavy strenuous exercises for about 4-6 weeks after this type of hernia surgery. It depends on your specific conditions, your specific type of surgery, & your surgeon's recommendations. You may be able to lift your child from a sitting or resting position at first, ask your surgeon.