Is it ok to take my toddler swimming when they have a cold/cough?

Yes. Swimmiing is fine. In general we can all exercise with a cold or cough. We should not press or work out hard as that can affect the immune system negatively. But swimming for fun with a mild cold is ok. The water should not be too cold either.
Cold symptoms. I think it is alright to resume normal activities when your toddler is back to acting normally. A little residual cough in that setting is not a big deal.