While it is, indeed sad that there are not more books aimed at youngsters, MtF and FtM, I have a feeling that, most children will pit the pieces together with what there is.
Introducing children to the essential concept that it's perfectly OK for anyone to express themselves in whichever clothes they choose is, I believe, enough.
I will be surprised if more books don't appear and almost every aspect begings to be covered. With that, there will enevitably be a lot of dross and questionable stuff.
But in the meantime,we can take advantage of the parcity of books by filtering what we show to children.
The only values we need to instill in children are those of tolerace of differences.