"Little boys do not have shirts."
Translation:Mali chłopcy nie mają koszul.
"Chłopiec" and "chłopak" are a bit different words, the first one denotes a younger boy (and this is the one used here, with the plural unmistakenly being 'chłopcy'), the rather will be rather used for a teenager or a young adult. Also it could mean "boyfriend".
As immery and mihxal have said, it gets pretty complicated with using "chłopak" in plural. But using "chłopaki", with its Nominative being not masculine-personal because of who the hell knows why, is still more popular. People do mistakes though - it should be followed by a verb in a form identical if as if you were talking about girls. So: chłopcy przyszli, dziewczyny przyszły. chłopaki przyszły, chłopacy przyszli.
Anyway, the change of -ły ending of an adjective in masculine singular to -li in masculine personal (masculine plural) is just one of the rules of Polish.