Hosp diagnosis gi bug in 3 yo, over now, but isn't eating much. output isn't matching input! no fever. no n/v. hard infreq bm w miralax (polyethylene glycol). time for dr visit?

Yes: The simple answer is 'yes'. Without a given timeline (when was the 'GI bug', last BM...) and specific details (what is the child eating/drinking...) it's difficult to provide much guidance. Your child's doctor will be able to ask these questions and more, they will be able to do a hands-on physical exam and better determine the best intervention if necessary.