Do you conjugate according to gender if the gender of the person is known? For example, would you say "Un beau enfant" if you knew you were referring to a boy, and switch to "Une belle enfant" for a girl, or does the noun always retain its grammatical gender no matter what?
My understanding of French is limited, but based on the commonalities with Spanish, I'll say yes, you conjugate based on it. If I'm wrong, frogs please berate me about it.
The thing is, enfant (child) is a "neutral" word, in that it doesn't have grammatical markers that indicate the gender. You just use gendered articles and adjectives around it, so if you don't know what the
enfant is or if it's male, you use masculine articles and adjectives, or feminine if you know it's female.
Enfant will then be understood to be masculine or feminine based on those other words, instead of remaining "neutral".
In your example, you correctly change Un (masculine) to Une (feminine). Same for Beau and Belle.
In Spanish, there's no neutral "child", you have
niño or
niña, but I'll use "bebé" (baby), which functions the same way as
enfant.
So to say "A beautiful baby", you'd say "Un bebé hermoso/Una bebé hermosa". Unlike in French, where the adjective is a whole different word, in Spanish the adjective just changes the last letter.
Of course, I think it's just one of those things where a particular word is suitable for only one gender, more than a grammatical rule; like calling a woman "handsome" would be considered insulting (the equivalent in Spanish would be "apuesto", you don't tell a woman she's "apuesta", maybe telling a woman she's "beau" rather than "belle" is also the same kind of insult).
Useful tool vs troons, that
