My DD was tested for this last year,I had taken her to our Gp as we had found a lump in her breast which she was saying was painful. Our Gp said he thought it was early puberty(she is 8 in March) and we had monthly appointments for a couple of months before he referred us to an endincrinologist at the local hospital.
At the hospital the consultant gave Dd a full examination and also x-rayed her left hand,they can tell the bone age of a child(this can differ from their 'actual age and is an indicator of early puberty) but although my Dd is tall for her age and growing breasts and having PMT symptoms ,the consultant is pretty sure that shes progressing normally.
The consultant said many girls this age will show signs of puberty ie breasts growing,pubic hair etc but its just fits and starts at the moment,if you get what I mean.It doesnt actually progress until a little later.
Having said that,when I was researching about puberty on the net,it says that puberty actually takes about 4-5 years in total with stronger sweat and breast budding being the first signs ,to getting your period being the final one.
I hope I've put your mind at rest abit,try not to worry about the sexual side too much at the mo,we just explained to my DD that her body was changing a little quicker than her friends,she seemed to accept it quite well.But I wouldn't burden her with the sexual side ,I think it would be too confusing for them the age they are,and put extra worries in they're heads,when they're already worrying about the changes being different etc.
Unfortunately puberty is happening earlier and many year 3/year 4 girls are having periods.