In spoken language, you can use "copil/copilă" to replace "băiat/fată" and i believe it is used in some parts of he country (correct me if i'm wrong), but the dictionary defines "copil" as "boy or girl in the first years of life". I wouldn't use it when referring to boy, as some might not understand you're implying gender as well.
Romanian here. Yes you can do that, but not the other way around. Copil means child (it can be either a boy or a girl). However, in Romanian, you can use baiat (boy) and fata (girl) even to describe a young person (like, let's say, someone who's 25). You can call them baiat or fata, but not copil, for that would be rude. Of course, now, you might hear older people calling them like that...
*Formerly ‘bastard’, probably from Slavic *kopylŭ ‘shoot, sprout; bastard’ (compare Ukrainian копил (kopɪl) or копиля (kopɪlʲa), Serbo-Croatian копиле (kȍpile), Bulgarian копеле (kópele)), from kopàti ‘to dig, cultivate’. Also borrowed into Albanian kopil, Byzantine Greek κόπελος (kópelos). Alternatively an old ultimately paleo-Balkan substratum word. Compare Aromanian cochil.
copil m (plural copii, feminine equivalent copilă)
The Slavic term is definitely native - its root *kop- means 'to dig, to delve' and respectively *kopylъ means a '❤❤❤', but metaphorically it stands for an "offshoot" (both of plants and of people). Therefore, it would actually be an insult to call some child a *kopylъ (only unwanted children should be referred in this manner)...
In this regard, why isn't a relation with the Latin "copula" /a bond/ considered here? This, in my opinion, makes more sense as an origin for a word for "children". Are there some phonetic rules that prohibit it?
Just because copil gained negative connotations in Slavic doesn't mean it had them in proto-Slavic or paleo-Balkan. There is nothing at all inherently insulting about calling a child an offshoot; that assertion makes no sense to me. That sentiment doesn't transcend the language boundaries.
Your suggested alternative is actually related to a different vulgar term pulă so I hope you weren't trying to distance from that :)
Well, the meaning "offshoot" is in quotes. In its full spectrum копuлъ actually means an unwanted appendix ~ degeneration. For people, it bears the meaning of both 'bastard' and 'mutant' (I couldn't find a word in English that properly translates it).
As a side note: When I dug a little deeper, I found that in (Present) Polish the usual word for woman ('kobieta') originates from the word for a mare and in the past it used be considered derogatory. So my assumption that negative words cannot possibly evolve to carry a neutral meaning may indeed be wrong.