If the Valar had children...

[Inziladun]

Well, the first problem is the Valar did not by nature possess real bodies. In The Simarillion their forms are likened to clothing:

Now the Valar took to themselves shape and hue; and because they were drawn into the World by love of the Children of Ilúvatar, for whom they hoped, they took shape after that manner which they had beheld in the Vision of Ilúvatar, save only in majesty and splendour. Moreover their shape comes of their knowledge of the visible World, rather than of the World itself; and they need it not, save only as we use raiment, and yet may be naked, and suffer no loss being.

Ainulindalë

In order to bear offspring, Manwë and Varda would have been obliged to take on permanent physical forms, like Morgoth.
If that did occur, the spiritual potential of the offspring should be the same as the that of the parents, as immortal Vala. However, being born into the world in a physical form should no doubt have some effect on his or her powers as a "divine" being; I would think they would not be as powerful as the parents.