Having 2 primary school children myself aged 8 and 10, they have been exposed and are aware of transgender people way before any formal teachings in school.
As such I don't see the harm in schools explaining that some people are different and shouldn't be deemed in a negative way for it.
However it's delivery in class is what has to be governed carefully and impartially.
In the same way as a child's political and religious views can be easily swayed by influencial people in their lives such as parents and teachers, I am sure at such a tender age so can their identification with gender.
I would take a very dim view of any teacher trying to sway my children's views to suit their own.
This happened in a secondary school local to me.
At an independent school namely Cheadle Hulme Grammar, the old head mistress was openly Lesbian which was fine.
However students from the 6th form were telling me how unfair they felt it was when a new common room was constructed exclusively for LBGT students.
There were also extra curricular events that were held EXCLUSIVELY for LGBT students such as talks from Sir Ian McKellen.
Whether it was this non inclusive promotion of views that resulted in a change of headmaster I don't know, bit the fact it was allowed to happen at all is a little concerning.