If you add a birthdate (or at least year) they will be ordered by by that.
Edit: Alternately (this is a newish feature that I hadn't used before), it you don't know the birth years, you can change the birth order by:
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Click on one of the children.
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Click "Change or delete" on the left panel
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Click "Change sibling order"
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Now you can move them up or down in the birth order
