"девочка" is a little girl, like 14 and under, while a young woman is "девушка" (googling both words for images makes it pretty clear).
So, this gets even more disturbing :P
Not a native speaker:
Because «девочки» is the genitive singular, which is used with numbers 2-4. «девочек» is the genitive plural and is used with 5 or more, e.g. три девочки, семь девочек.
"Кот" is a (male) cat in nominative singular
"Кошка" is a (female) cat in nominative singular
"Кошек" is (female) cats in genitive plural
If you don't know the sex of a cat you can say any variant that would you like. it refers to almost any animal.
Why is кошки used sometimes and кошек some other times? Both are for more than one cat, so why the difference?
When you use numbers in Russian, the case of the counted noun depends on the last digit of the number. There's a detailed breakdown here on RussianLessons.net, but basically:
For numbers ending in 1--but excluding 11--the noun takes the singular version whatever case it would otherwise have. (один will also decline like an adjective. двадцать одна кошка видели меня, "Twenty-one cats saw me", but Я видел двадцать одну кошку, "I saw twenty-one cats.")
For numbers ending in 2, 3, or 4 (except for 12, 13, and 14), the noun takes the genitive singular, regardless of its role in the sentence.
For numbers ending 5, 6, 7, 8, 9, 0, and all -надцать numbers (11-19), the noun takes the genitive plural, regardless of its role in the sentence. The "-надцать" rule takes precedence over all others; whether it's 11 or 111 or 1011 or 982350982711, they're all considered to be ending in 11 (одиннадцать), which means they're all -надцать numbers and they all make the nouns they're counting take the genitive plural.
Девятьсот восемьдесят два миллиарда триста пятьдесят миллионов девятьсот восемьдесят две тысячи семьсот одиннадцать кошек
Девочка is feminine. два becomes две for feminine nouns, it just works like that. Like Один becomes одно for neuter and одна for feminine (Similar to Spanish "un gato" vs "una gata").
I very strongly recommend you to use wiktionary, it has been an invaluable tool in my learning so far. You can check all forms of кошка here:
There you can see that, one is genitive plural, and the other nominative plural.