I didnt know seeds had sexes. If this is right, how do you tell the difference ?

He he - good question! It would be easier to think of the grown plants rather than the seeds, as you cannot tell the sex of a plant from the seed. A few plants are either male or female - Skimmia is one...I think Holly also and Ginkgo biloba - the males bare flowers that produce pollen while the female plants produce flowers capable of collecting the pollen from the air. So, to anwswer your daughter's question, you have to look at the differences between the flowers to tell whether a plant is male or female. If the plant has fruit on it, then it's a female. Of course, this only applies to plants that DO have different sexes - I'm sure there's a proper term for this but I've no idea what it is! Someone here will know :-)

When plants have female plants and male plants I think they are called dioecious

Some plants are also hermaphrodites, having both male and female flowers on the same plant, and some plants can even change their sex.

Well done Gilli :-)

then there are the ones that have separate male/female flowers they are monoecious. things like begonias, some willows and some hazels. I think nettles are too.
clever things plants.