not exactly sure, but this reminds me of the book "Kindred" by Octavia Butler. it takes place in the early 1800s where a young black women who is married to a white man (where they live in the 1970s), some how goes back to the past to when her great great grandparents exist (the early 1800s) to keep her family chain continuing. however, she discovers her great great or just great grandfather is white, but loves a black slave. the slave does not love him but he forces himself on her (rape) many times because he thinks he can make her love him. however he often gets in trouble (deadly things), and the black women from the 70s has to keep him alive until he gets the slave pregnant to have a child that will become the 70s women great grandmother and keep the family chain alive. but every time the white guy gets gets in trouble, she gets "called" back to the past. but every time she reaches a near death experience ( while she is in the past) she goes back to her time. and this keeps happening unitil her great grandmother is born.
the white guy and the black slave were friends since they were little, but he wanted more with her, which makes the book interesing because there was no shame in raping a slave, but there was shame in being in love with one.
i know this is completly differet from what you wanted, but it is an extremly good book i recently finished reading and cant shut up about so i have to do so many things to release it from my mind.
however in answer to ur question, it depends on the family if the kids could mingle or not. im pretty sure they could, but not all because the 30s was way before the civil rights issues grew to a much more serious issue in the 60s.