I can well understand why families would not want a relative to be described as illegitimate as this word suggests unlawfulness when there is nothing unlawful about having a child when not married and so I am happy to have 'natural' as an alternative.
I therefore suggest it is left to the family researcher to decide which if any of the two adjectives are used to qualify the pedigree. In earlier years only illegitimate would have been used but in this modern age it is good to have a another way of putting it but I am not sure how many readers will understand what is meant by a 'natural child'.
Note that these adjectives only appear in a report when the Pedigree Link custom tag 'Qualification' has been used.