Unfortunately, the problem created by this is what happens when a female sovereign and a male sovereign set up an arranged marriage. Now who goes to the other side? Under your rules, a female-female negotiation and a male-male negotiation has no issue, in either case the child with the same gender as their sovereign stays with the faction. But how would you resolve the issue when a male sovereign negotiates with a female one? The male wants his son to stay with his faction, the female wants her daughter to stay with her faction (or, vice-versa, the male expects his daughter to leave his faction, the female expects her son to leave).
I think a system based on lineage to the throne would make more sense (2nd in line daughter stays with her faction if marrying a 3rd in line son), but that still creates a tie-condition that must be resolved. The whole system is complicated by the fact that channelers are basically invaluable; there is no amount of any resource that really compares to the value of a channeler, so basically the only viable deal is a child-for-child trade.
