Masculine personal plural when referring to children
I've been to Poland recently and noticed that when a woman referred to her niece and nephew (aged 3 and 4), she used the masculine personal plural form. That happend when talking about them, as well as when talking to them.
Is this a new language trend? Has it always been like that?
Personal plural form is the same for mixed gender groups as it is for all male groups. It's only different for all female groups.
I know that, but according to grammar textbooks, children are generally referred to as "one" instead of "oni".
Absolutely not. Well, maybe if you are making some hypotherical, generic statments about children in general, though I can't really think of an appropriate example of that. Absolutely not when you are talking about specific children.
Exactly. The noun "dzieci" itself is 'not masculine-plural', if you decide to use it. "Dzieci są jeszcze za małe". But if those children become "Ania and Adam", then you have "Ania i Adam są jeszcze za mali".
About 'talking to them'. You can imagine "Dzieci, jesteście za małe!" ("dzieci" is Vocative here), but unless you use it explicitly, it's "Jesteście za mali!"
Also the way I would explain it is that "one" (neuter form) is used, when you don't know kids' gender, so for example if you see a lot of babies in a hospital, you could say "one są takie małe".
But once you know the kids' gender, then you use "oni/one" when you talk about them, so if it's a nephew and a niece, it will always be "oni" as in "oni mają tyle energii"