At this point, my childhood collection is mixed in with my regular collection. I’ve even replaced some LP-HP cards from my youth with EX-NM cards I received as gifts with other orders once in a while. 
For context though: I had about 1k cards in my youth’s collection, and therefore not all are noteworthy or sentimental to me. There defintely are some cards I would never get rid of though, but even those are mixed in with my general collection for the most part.
The most noteworthy ones:
The Moltres promo I received at cinemas as a kid, my favorite TCG artwork. I graded it for nostalgia sake a couple years ago:
The first Pikachu I pulled as a 7 y.o. kid is in my Pikachu collection (I also have a NM copy of the same card). It’s an unlimited edition Jungle Pikachu, and I wrote a 1 on the back to mark it as my first Pikachu card ever, since it was HP already anyway (even though I hate HP cards with text written on them - unless it’s a signature - and this one I did myself.. 
).
The first holo I pulled as a kid (Base Set Chansey) I traded away soon after (I cared more about quantity than holo cards, so a 3 for 1 trade was a no brainer for me at the time ). But the second holo (or third, since I opened two Jungle booster packs at once), I still have, which is also still one of my favorite cards from the Jungle set because of it.
I’m just gonna quote this old post of mine, which also included the fourth card I wanted to mention anyway:
Also, I keep a checklist of every non-duplicated card I own (little over 16k unique cards rn apparently), and you may note some grey rows in that document. Those are all cards I took over from my little brother’s youth’s collection. We made an agreement that if I’d ever sell one of his cards, I’d give him 25% of it. (And yes, he knows I almost never sell, especially not from my non-duplicated cards. It did include a Shining Gyarados; Shining Steelix; and Crystal Nidoking, FWIW. I’ll defintely give him 25% iff I ever decide to sell those.)
Greetz,
Quuador