I don't grieve for not being born cisgender, since I think I would have been an entirely different person then.
I do however grieve for not coming out once I realized that I am trans, when I was around 13-14 years old. I grieve for not having had the opportunity to go on puberty blockers (my puberty had barely started at that time, it was late and slow due to me being hormonally different; at least this wasn't discovered until the pre-HRT checkups, as I'm sure they would have pumped me full of T if they had found out during my teens), I grieve for not having had the opportunity to start HRT in my late teens, and I grieve for remaining in the closet and having let myself be a suppressed, miserable person throughout my twenties and into my early thirties.
I also grieve for compensating for the fact that everything about my sexuality felt "wrong" (I'm bisexual, but my attraction towards men felt heterosexual and my attraction towards women felt homosexual, meaning all my fantasies were "reversed") and added to my dysphoria, by falling for religiously conservative arguments telling me that they were wrong. It became an ecco chamber with my dysphoria feeding the sexual conservatism and the other way around, and it left me with the experience of a fourteen-years-old in the relationship and sex department, as a 33-years-old. And now I feel like I'll never find someone to share my life with, which sent me into a depression earlier this fall - I'm thankfully feeling a lot better now, and at least I know that the reason I can even grieve over this is because I in general feel so much better, so much less dysphoric, that I'm finally able to access my grief over my history, since I now know that it could have been different. Also, in general, I've been a lot more in touch with my emotions after I started HRT. And perhaps that will help me move on, and get past these issues and finally start living my life.
So yes, I grieve, but by "losing my childhood" I mean "not transitioning when I first realized". I can't even imagine being born cis, and in a sense, being trans has taught me so much, and essentially made me into a better person. Of course, I could have been a good person also if I were cis, but I would have been a different good person. And even though my life has been, and still is, challenging, I still wouldn't trade being me for being someone else.