How old are the little ones? You can get a magnifing(sp) glass and look at them through that, females will have a tiny gravid spot...if they are older then the Males fin will start to point... Other than that its the only way I know of to sex them
I don't even try to sex them until they get older. They just move too quickly. I just look for the pointed fin. If they have it, they're male, if not they're female.