Before I was a WAHM I worked in medical media relations/PR and spent a lot of time reading and analyzing medical research.
I agree that many consumer products contain things that a human body may have never been intended to come in contact with. However, I also know that when looking at scientific research, you can't draw conclusions about ANYTHING based on one single study. In the grand scheme of things, that one study simply means that in a particular group of people, using a particular group of variables, etc., there "appears" to be a link to something. Real conclusions, medical guidelines and recommendations don't come about until a "preponderance of the evidence" happens. That means that gobs of research on lots of people of different races, gender, socioeconomic levels, environments, etc. point to the same conclusions.
I haven't found the study you're talking about, but you would want to ask if it takes into consideration these children's diets, family history, environmental factors, etc. There is more "poisoning" in most of the food we eat than in what we wash our hair with. I'd be more concerned about feeding my kid a hot dog than washing him in pleasant-smelling baby wash.
Also, the fact that the purpose of our skin is to keep things from getting into our body would indicate that it's pretty hard to get major hormonal changes from the highly processed lavender scent in a shampoo (probably can't say the same if you were to roll naked in a field of fresh lavender every day for 5 years, but no one is going to test that, are they?)
Chances are that this was not a randomized, double-blind, placebo-controlled trial (the so-called "gold standard" for research) of the effect of lavender baby shampoo on a baby. I'm betting that it was an observational study with limited reach.
So, really, the only concern you should have about your baby bath is whether or not your baby gets clean from it.