I’ve often wondered about certain ‘difficult’ (to a foreigner) sound aspects, such as 'trż', which, as a somewhat subtle sound
I don't think there is a Polish word with "trż" in it, but there is the postalveolar affricate [t͡ʂ] (usually <cz>) and the sequence [tʂ] (most often written as <trz>). I suppose small children do have some tendency to mix up the affricates and the sequences of a stop and a fricative, but their distribution in the normal adult speech doesn't totally reflect the orthography either, for many speakers including me.
And small children often (even stereotypically) mix postalveolar and dental sounds (usually pronouncing them all as dental), just like some Anglophone children tend to confuse the dental (/θ ð/) and labiodental (/f v/) fricatives, from what I have heard. The phenomenon is called
seplenienie.
/r/, which is normally realized as a trill (although somewhat "weaker" than the one in Russian, in my opinion) or a flap, is another difficulty for many young, but not only, native speakers. In fact realizations such as [ɹ], [ʁ], [ʀ] (and even [l] etc. for small children), are relatively common (so common that I sometimes wonder if it won't ultimately lead to some sound change
), though they're regarded as speech impediments. My own /r/ tends to be some weird velarized flap, although I'm able to produce a normal [r], I do it only when I am not understood.
Also, do young children often make mistakes when using the past tense, since it is gendered, for example a girl saying "ja był...", or is this something they pick up without having to be corrected?
Hm, neither I'm an expert in child language acquisition, nor I've had some enormous amount of contact with little children, but I've never heard a child confusing the masculine and feminine forms when talking about themselves. Perhaps they pick it up early because such errors would be strongly socially stigmatized, or something, I don't know.
I have noticed that I sometimes stumble before using a 2nd person plural non-virile form in -łyście (e.g. byłyście, znalazłyście), I suspect it's because these aren't used that frequently, because they require a group to consist solely of women; the virile variants are more frequent and safer in many situations.* Well, maybe I'm just weird.
*In addition, as a man, obviously I never use the 1st person pl non-virile forms except when quoting some female.