The chlorine won't hurt them from a chemical standpoint but long exposure will remove the oils from their feathers. So a little chlorine won't hurt but daily would leave them a little bedraggled.
I would be concerned about the long term effects of the ducklings drinking chlorinated water on a regular basis, not to mention chlorine can irritate the eyes and skin of animals. I think it would be best not to use chlorine in the pool at all. IMO.