So, I got some tree seeds for Christmas. I planted them, intending to grow them as bonsai. Recently, they were about an inch tall. When I repotted them and I put them in a bonsai pot, they suddenly started to wilt and die.

I feel so sad. I feel like I killed my friends, the little trees.

What did I do wrong? Why are they dying?

Species were bristlecone pine and giant sequoia.

Roots too young and tender to handle?

Well it wasn't really sudden. They quite probably died because they were way to small to transplant.
It is a valuable lesson to have learned. Don't give up. Try it again!

Agree with Bill. Way too early for the bonsai pot. You want to grow them in a regular pot for about three years, regardless of those what those "instant bonsai kits" claim.

Also, the giant sequoia probably wasn't going to survive anyway, so you saved yourself later heartbreak. Some sprout, few survive in pots or outside of their natural habitat.

Thank you. I will try again soon.

why not just sprout them in the pot you want them in so you don't have to move them?

If you are growing trees as bonsai subjects the main thing to remember is that they are trees - not houseplants. They need to be outdoors. As others have said they can stay in the same small pot for a few years before you start to train them.