Talking about intimate body parts and bodily functions is always a tricky decision: We generally have a choice of medical terminology (look in a dictionary) vulgar and crude language ( I'm sure you know these words) baby-talk (as in your question) coy euphemisms ( e.g. 'number twos') You'd use the first to your doctor and the second (perhaps) among close friends. In most other situations, the third is probably the least inappropriate option. So yes, informally, if you don't want sound vulgar, pretentious, or coy, then sometimes adults do say 'poop' (or 'poo' in GB English) . If none of the other words seem right, then sometimes baby-talk is the best choice.

I think it highly depends on culture and personal preference. Since the dictionary you are citing seems to be a US dictionary, it seems "poop" is commonly used by children in the US culture. I personally use "poop" rather than "shit" or "dump" merely because I prefer the sound of it. However, the adults (and even the children) in my culture use "shit" more often.