In some families, it can be difficult to determine the order in which children are born. The problem is exacerbated in places where there are no vital records. Children may be listed in wills in order or age (or they may not). Children all signing off on a quit claim deed after the surviving parent dies may be listed in order (or they may not).
And always ask yourself "how crucial is it that I know the birth order." Sometimes it is and sometimes it isn't--it really depends on the family and what records are available and how detailed they are in terms of relationships.
If you've assigned children to a family in an order that you are not certain is the birth order, make a note that effect in your genealogy database.