Nikhil didn't know he was a Father to 2 more kids Badal & Gagan as was evident in the previous episodes.
Probably Pallavi got to know after the divorce she was pregnant & never informed Nikhil.
I think Nikhil was given the custody of both the elder kids since he was well-off & the expenses as kids grow old keeps on getting high.
Barkha was left with her mother cos she was very small then.