My sister recently asked me a question which I’m going to refer on to you: In the Old Testament many righteous men had wives and concubines. If the concubines were considered lesser wives, were their off-spring children “not of the covenant,” those souls who would not accept the plan, anyway, but needed bodies to come to earth?
You are right about the status of concubines in biblical times. We read from Daniel H. Ludlow, in A Companion to Your Study of the Old Testament, p.11, the following excerpt—
“The biblical definition and usage of the word concubine is not the same as current dictionary usage. A current dictionary defines a concubine as ‘a woman living in a socially recognized state of concubinage’ and then defines concubinage as ‘cohabitation of persons not legally married.’ (Webster’s, p. 232.) In biblical times, a concubine was a legally married wife, usually of a second class status either because of social order, economic position, race, or nationality (a foreigner). Concubines had legal status and rights as wives, although not always of the same order or magnitude as the rights of “first wives.”
With respect to your question with respect to the children of the “lesser wives” being covenant children, I can only express a personal opinion. My opinion is that since their parents were legally married the children would be heirs to all the blessings of the gospel for which they and their parents were worthy, regardless of any social restrictions placed on the status of the mother.