Sexing rats usually isn't that hard, I have sexed thousands of them over the years. It is easier if you check them before the hair fully grows over their belly's, as you can easily identify the nipples at this time. It is usually easiest around the 2 week mark.
Females will also have an evident line between their anus and urethra.
Its a bit hard for me to make definite identifications from the photos.
Feel free to message me if you need any more assistance.