i think nandini would have given birth to a son in jail.after that,tanya would have secreatly given this kid to mandira.she has two big reasons for this;
this child is nandini and karan's son whereas bhumi is nandini and ansh's daughter.so,tanya is the child's STEP MOM.so out of her jelousy towards nandini shez done this and may be mandira has influenced her to do so.
if this child disappears then manthan will have the whole share of karan's property.so,i think its tanya and mandira's plan.mandira too wanted her grandson to be wid her.
i think mandira has neglected this child for which he's rude and indifferent towards reletions etc.later nandini will make attempts to make him believe in relations and he will turn into an adarsh beta of virani pariwar,like gautam,sahil etc etc