Posters saying this baby and his family couldn't have died in an airstrike because of the ceasefire
Bombing immediately before the ceasefire was more intense particularly in one area of Khan Younis, this being the town but not necessarily the area where this family were allegedly being held captive by JIP terrorists. Not sure that Hamas knew their exact location?
It's more than possible that the JIP captors [who Hamas say they had already lost touch with/having a communication breakdown with] were injured or killed at the same time.
In the chaos following an airstrike it can take time to retrieve casualties and identify fatalities so it's possible that it took ?a while for Hamas to find out what was going on and gain possession of the deceased hostage family. Hamas rightly would be reluctant to declare the family had died until they had proof
Hamas are offering '7 women and children and 3 bodies' in the hostage exchange today so we'll maybe know more later 
Personally I doubt if anything will be completely clear now or in the future