Good on you for setting up the Jellyfin early, it's still on my to-do list
My personal favorite childhood movies/shows that made a real impact:
Fern Gully, the Disney animated originals (not remakes) mentioned elsewhere in the thread, Nightmare Before Christmas, Princess Bride, Neverending Story, Star Trek 4 (the whales one), Toy Story
Star Trek TNG and TOS, the old school B/W Addams Family, OG Looney Tunes, Nature on PBS, Nova on PBS, Mr Rogers, Arthur
Additional stuff I'll be adding to my own kid's Jellyfin (when I get to it)
Avatar the Last Airbender, Kipo and the Wonderbeasts, She-Ra:PoP (the Netflix one), Bluey, Storybots, Puffin Rock, Lucas the Spider, Trash Truck, Ms Rachel, Daniel Tiger, Elinor Wonders Why