When I add children to a set of parents the first thing I do is set the birth order of each child, if known. Sometimes I know the birth order, but not the actual dates of birth, except for maybe a few of the children. When I add the known dates in it always moves that person up in birth order, even though that order was already set...then it must be set again to get them back in proper order. This seems like a bug to me.
Also, in the case of twins, while sometimes the birth order is known, usually it is not...yet, we must specify the children in some descending order. Why not allow twins to be set to the same birth order, then any subsequent children as their actual number, for example, in the case of twins and then two more single child births: 1-1-3-4. If twins were highlighted somehow would also be informative.