If we wanna put it simply, Kalyani doesn't see anything beyond her Pillu and Malhar isn't on the same wavelength on that issue... Kalyani is his biggest weakness and, no matter how hard he tries,when having to choose between letting Kalyani go or risking losing Moksh, Malhar can't let Kallu go even thou he loves his son to the moon and back.
That's something Kallu can't understand,she has proven time and again she can sacrifice anything and anyone for Pillu (and Anu).
What Malhar sees that Kalyani doesn't,though,is that Moksh is so attached to her,he wouldn't be able to live without her,so all her talk about sacrificing her life for him are nonsensical.
This also highlights Kalmas different ways of thinking,as seen before: Malhar considers sacrificing one's life for someone else a silly act,he told as much to Kalyani during their Mumbai trip where she ended up with a bomb attached to her.
Malhar has also said how he wouldn't be able to raise Moksh alone... he wants both because,I guess, he can't function without her in his life... jaisa beta, waisa baap I guess!