It's because of an annoying (but simple) rule. Look at the final vowel of the word. If it is e or a dotted vowel (i, ö, ü), go with -ler. If it is a or a dotless vowel (ı, o, u), go with -lar.
Çocuk's last vowel is u, which is dotless. So we say "Çocuklar". If it was "çocük" (which doesn't exist), it would be çocükler.