Hi,
In my opinion , this would be clubbed with the Income B , since this is not received by B's son by way of any manual work or activity involving application of skill, talent etc.
Extracts of relevant paragraph reproduced :
[(1A) In computing the total income of any individual, there shall be included all such income as arises or accrues to his minor child 96[, not being a minor child suffering from any disability of the nature specified in section 80U] :
Provided that nothing contained in this sub-section shall apply in respect of such income as arises or accrues to the minor child on account of any—
(a) manual work done by him ; or
(b) activity involving application of his skill, talent or specialised know-ledge and experience.
Regards
Hariharan