Please always advise what version of FTM for Mac that you are using?
I will guess that you are using FTM for Mac 2 that has the blended families feature.
Blended families and Children sort both work on the same data items, so to get the children's sort to work, you must first temporarily turn off the blended families.
Then sort your children either with the sort button or the up and down arrows
Note the Data Errors Report can find all children out of birth order, Errors to Include.
When you have sorted your children you can turn the blended families back on
You do not have to remove and re-enter and it is definitely NOT the current way to do it.
See also http://ftm.custhelp.com/app/answers/detail/a_id/5440/kw/Sort...