"Les petites filles lisent un livre."
Girl already implies youth, so in this case it would be about their size, I translated it as small girls and that is acceptable, as is little girls.
The question before this for me asked for translations of "young boys", and said I missed "petits garcons", so why doesn't "petites filles" count as "young girls"? Is this question wrong, or was the other one wrong, or is this an exception?
petit = used for masculine nouns
petite = used for feminine nouns
Add an "s" to either for plural
There is an audible difference between "petit" and "petite". There isn't really an audible difference between singular and plural, unless the word after petits/petites starts with a vowel...then you get a "Z" sound between the two. Like "Les petites amies" would sound like "les petite Zamies". I don't know if I explained that clearly. Hope it helps somewhat!