The basic form is "mały", so there's a "ł" already. I can't explain why mały becomes mali, but it seems it's common for in mixed plural form (not sure if it's the correct name)
- duży chłopiec → duzi chłopcy; duża dziewczynka → duże dziewczynki
- sprytny (smart) chłopiec → sprytni chłopcy; sprytna dziewczynka → sprytne dziewczynki
- miły (nice, kind) chłopiec → mili chłopcy; miła dziewczynka → miłe dziewczynki
There was a great comment on "Źli ludzie." by HelioLBS which explains the l-question: "Well, in Polish (and other languages) there's what is called "a phonological agreement". Soft consonants go with soft ones and hard consonants go with hard ones. If I'm not mistaken, "l" is considered "soft" and "ł" is considered "hard". Nominative, singular, masculine adjectives usually end in "-y", which agrees phonologically with hard consonants, for instance "zły". To form its plural, we need to drop the "-y" and replace it with its soft version "-i", but the latter only agrees with soft consonants, so we need to drop the "-ł-" and replace it with its soft version "-l-", but "z" is not soft, so we also need to replace it with its soft version "ź". Then we are left with "źli", in which all three sounds agree in phonological softness. Please, everyone, feel free to tweak this explanation; I'm not a 100% sure I explained correctly."
It's more the opposite question you should concern yourself with, why does "ł" become "l" in the masculine form, because "ł" is in the basic form of the adjective "mały" (the basic form given is the masc. sing. Nom.).
"The masculine personal plural adjective ending is -y/-i and the preceding consonant is softened: dobry → dobrzy, ładny → ładni, miły → mili, wielki → wielcy, drogi → drodzy; for more examples, see below.
Swan, Oscar (2008-10-12). Polish Verbs & Essentials of Grammar, Second Edition (Verbs and Essentials of Grammar Series) (Kindle Locations 780-782). McGraw-Hill Education. Kindle Edition.
As you can see the third example is almost exactly the same word, save one vowel change.
Without explanation it feels like puzzeling in stead of learning a language. So puzzeling I find that the adjectives before nominative plural nouns are formed like so: -y/-i for male personal, and -e for male non-personal, female en neuters (not taking in account some some extra letter changes) Can anyone confirm or correct?
It would also be a nice timesaver if we could get a hint on the adjectives for plural nouns in Accusative and Instrumental.
In plural you don't have masculine/feminine/neuter. There are 'only' two plurals. Sometimes they are called masculine and feminine, but although that seems simple and tempting, it's just not true. Their real names are complicated, but at least descriptive.
The first one is 'masculine personal plural' - generally, it's used for 'groups of people including at least one man'. So as "chłopcy" are men, they are obviously masculine personal.
The second one is 'not masculine-personal plural' - and it's used for everything else. All feminine nouns, all neuter nouns, and the masculine nouns that don't denote people.
The only potential reason I see is the problem of meaning dissonance. If you use "dziewczyna" then you imply that the girl is not so little anymore, so "małe dziewczyny" sounds a bit weird.
You don't learn many diminutive on Duolingo (in part because including them would exceed the limit of possible answers in many questions), but some of them, like "dziewczynka", are so important that they are taught here.
Mała dziewczynka, to be accurate, should be translated as "a small, young girl", which isn't accepted as an answer. I wish Duolingo would use dziewczyna when doing those exercises. Dziewczyna and dziewczynka, I don't believe, are interchangeable , yet are translated here as if they were. I think most of us know dziewczyna by now.