All of the tribes are created using the same model, and they use mmo character style sliders to stretch and squash that one universal model into the different tribes. Each tribe has different constraints for the sliders, so a brudvir has a different range of overall body sizes than a kypiq does, for example. Both are still on the same slider, though, it's just that they are constrained to different parts of it.
When a child is created, it gets its own ranges for the slider based on both parents.
With a kypiq-brudvir hybrid, this could get a bit interesting because of the massive difference in size. Maybe it would split the difference, or maybe each child would swing wildly in either direction.
We do know that the tribal specific abilities will be either be tied to a certain slider, or have their own slider. The Janoan ability to swing arm over arm through the trees is tied to how long their arms are, while things like environmental resistance have their own slider. Your kids will be more resistant to cold than a full kipiq, but not as resistant as a full brudvir.
There are also traits that you either inherit, or do not inherit. The brudvir canines are one of these, some of your children may have them, and others will not.