Bonsoir, Yes, "petit ami / petite amie" sounds kinda childish. If you are not engaged, and instead of "partner" (more used for business associate) you can say : C'est mon compagnon / C'est ma compagne. (companion) or : C'est mon homme / C'est ma femme. Even if you are not married, it's truly common in France. " Mon chéri / Ma chérie " , " Mon amoureux / Mon amoureuse " is more colloquial, you use this wording between friends, and close people.

In Canada, you definitely wouldn't hear people over 5 years old saying "petit(e) ami(e)"! We say "ma blonde" (my girlfriend) and "mon chum" pronounced TCHOM (my boyfriend). You can also say "mon/ma partenaire". We would not say "ma femme" if not married, nor "mon /ma fiancé(e)" if not actually engaged, but you could say "mon homme" (my man as it doesn't imply marriage).

Actually, the most common expression would be: "mon ami(e)". If there's nothing else that define that person (ex: "mon ami japonais"), people will assume that he's your partner ("mon" makes that person unique, and since, as I said above, no other information has been added, that person is unique to you because it's your boyfriend). For that very reason, if you have some activities with someone who's just a friend, avoid to use "mon ami(e)". Use, "un ami / une ami" instead to avoid ambiguity. "J'ai diné avec un(e) ami(e)" :) ps. I agree with the other answers