When the promo came out , that was what I thought too. I even rewind the scenes many times to see if there is any possibility that the twins are changed. But i think sadly its not the case as you see the child in jo's hand doesn't have that black dot ,but the one in ruq's hand had it.Remember that is the way even before ruq identified hussain.So unless its just a blooper i see no chance of child being exchanged though i wished it was the case ,So for now i believe its hasan who is poisoned ,
My take on the precap would be
hassan would be saved for time being by hakima which would confirm everyone that the fire was not an accident and like MD said it was a trap to conspire against the babies .
So may be he might say to ruq for timebeing the children could be together till they find the culprit. As Ruq always equate child to MUZ may be she might refuse initially but may be give in .Upon this Zennat may instigate her against Jo that it could be just jo's drama to get the child back as no one would have been aware of the plan and might they both join together as co-conspirators but with a different agenda