Cooties (occasionally spelled coodies) is an imaginary disease that little boys say little girls have.
I'm French. The best translation I can think of is (as, apparently, "cooties" is an insult boys use against girls) "les filles c'est nul" or "les filles ça craint" (not "ça craint = it scares" but "ça craint = it's the shame of being with, it's boring". Literary translation would be "les filles c'est des pouilleuses", "the girls are dirty / verminous persons", but it is not something children say in real life. This is too "nasty" and saying that would probably result in the children being scolded by their parents; saying "les filles c'est nul" might make parents smile.