If your parents are conservative in terms of substances and for them grass is the first step to heroin, then it is definitely not worth talking about, you will only create problems. I've made hints to my mother that substances are better than alcohol and that I am inclined towards such experiments, she at first perceived all this as a typical conservative, but later her position on this matter shifted somewhat and she realized that everything was not so bad.
Parents, perhaps, should not tell about this, unless the parents are notable for their wide openness to the world. In general, there is nothing wrong with MDMA as such - it is not heroin. But it will be very difficult to explain this to parents.
And again, the main question is, why tell parents about it? Share experiences? They are unlikely to understand this, they have never had such an experience.