Those are still abit young, probably at least 1 to 2 years away from breeding. Once they reach maturity, sexing is real easy.
These are pictures of some of my breeding group. They are around 10cm in size.
A nice male.
Same male with a close up on the body.
Here are 3 of them, 2 top ones are females, bottom left is a male.
So, in conclusion, yours are just abit on the young size. Give them a year and maybe...maybe you can tell.