My personal opinion, (and I'm no medical professional) is that the biggest concern about babies in the pool is from visitors. If you are changing the baby's diaper, you've already been exposed to any pathogens that are there, and likely you may have given them to the child to begin with. The real concern is with parasites like cryptosporidium and giardia, which are easy to pick up, even in a well sanitized pool, and can be a health risk for persons with reduced immune system function. It wouldn't hurt to elevate the chlorine a couple of ppm before they swim, then nudge it back up after they swim. Urine is no real risk, but it will consume chlorine which can reduce the sanitizing capability of your water, which can then put you at risk of other pathogens.
My bottom line (no pun intended) is that I allow my grandchildren to swim in our pool, but I won't swim in a public pool with young children. Actually, I rarely swim in public pools at all anymore. The water always looks and smells bad compared to mine.