In that case, I think the chicks probably do have the Brahma father, because I would expect the White Giant rooster to produce chicks that look black instead of showing a pattern of Silver and black.
With a Light Brahma father, I do not think there is any good way to tell if the mother is the...
Do you know what color eggs they hatched from?
If they have a BCM mother, and they have pea combs, and you only have the two roosters in the listing, then yes they would have to have the Light Brahma father. Two parents with single combs would produce just chicks with single combs.