Here the problem is more about Pooja and Ganga rather than Mannu and Pooja.
Pooja's fixation looks more like obsession and Mannu in his own way likes her but owing to his very young age he doesn't realise that he is encouraging her.
Ganga's handling of Mannu needs a different approach too .Here it looks difficult because Mannu doesn't really know what he wants from Pooja Talking to him abt Pooja may lead to putting thoughts into his head which didn;t exist in the first place.
Then Ganga's own problems in life which again seem to prevent her from thinking properly and doing the right thing.
It's better she talks to J about Mannu and also have a talk with Sarita.
They should be discouraged from spending too much time with each other.