Ok, I spent a little bit of time pulling together some links and data (for those who want to dig into this).
Eo's and children: in the US (don't know about other countries) it is not legal to conduct medical studies on minors except under certain conditions. There simply isn't enough credible scientific evidence out there to responsibly declare any essential oil safe for children.
That's pretty much the problem (as it were). I know here that the amount allowed in natural lice treatments was lowered (I can't recall from what to what, but it was on the basis that it was too strong a solution - this treatment being often used on young children of primary school age). It is a ridiculously effective treatment, and one round with tea tree will kill all adult lice and most eggs, and another round a week later to catch the eggs that survived and hatched, and the job is done. It's that good at killing insects.
Anyway, this is what I found for you:
Just to be clear, we are talking about Melaleuca alternifolia leaf oil (black tea tree is pretty toxic), specifically in the context of the effect of topical application on human hormones (only).
On one side:
Tisserand refutes the idea that tea tree is capable of crossing the skin barrier, despite it having “weak in-vitro estrogenic action in MCF-7 cells (Henley et al 2007; Nielsen 2008) on page 443 (of 2nd ed etc.)
He also questions the veracity of the reports, and suggests that there was one case of a 10 yo boy, not 3 boys, and stated that a website suggested that the testing was done by a competitor. (same page, paraphrased)
There is a substantial amount of information contained in Tisserand's reference book; tea tree information is contained across 6 pages (pages 440 to 445), however for this particular topic, he discusses reproductive toxicity across 2 detailed paragraphs at the end of page 443 across to page 444.
On the other side:
This page from the National Institute of Environmental Health Sciences seems to name the doctor who treated the 3 boys and where the study was published (in 2007) (I cannot see that Tisserand has referred to it, but I also have not checked his source material):
https://www.niehs.nih.gov/news/newsroom/releases/2007/january31/index.cfm
This page makes reference to it needing further study, that the tests were in-vitro and the boys had no other identifiable reason for their condition (calling the idea speculation), so they are not drawing concrete conclusions at that point.
Under the UN’s global Harmonised System of Classification and Labelling of Chemicals (GHS). Tea tree is classified as:
GHS Classifications:
Flammable liquid, category 3
Acute toxicity, category 4
Mildly irritating to eyes, category 2B
Irritating to skin, category 2
2015 publication (possibly the same 3 boys, 2 using cologne, 1 tested cologne with lavender), says in conclusion “Exposure to estrogenic substances, such as lavender, should be explored in children presenting with prepubertal gynecomastia/thelarche.”
https://www.ncbi.nlm.nih.gov/pubmed/26353172
Links to a lot of research (over 1,000 papers on tea tree oil):
http://www.attia.com.au/search_abstracts.php
This is interesting (related):
https://www.ncbi.nlm.nih.gov/pubmed/23211454
So, at this point in time,
a) Tea tree and lavender can affect human hormone balances when applied to cells directly
b) There a very small number of cases where a hormonal effect is claimed to have resulted from topical application
(I have found a maximum of 7, or possibly 4, being either 6 boys and one woman, or more likely 3 boys and one woman)
c) The dataset is too small at this point in time to come to concrete conclusions, ongoing/further monitoring and reporting is required.
