Repotting a small tree, do I have to?

[MallorieArcher]

Full disclosure, I suck at gardening and have killed everything I've tried to grow!
Last year we bought a small apple tree that was already growing apples from a garden store and potted it into a new slightly larger pot, about 12 across and round.
We bought it to bury DD's much loved small pet as we don't have a garden we could bury him in, the pot sits on a patio and is doing ok.
My mum has suggested repotting so it's okay over the summer, but, there is a dead animal in the pot! Can I feasibly leave it in its pot for a few years without it dying on me and traumatising DD further? Any advice?

You bought it in a pot, and now it’s sharing a 12” pot with a small corpse? Consider the corpse as compost and get both in the ground as soon as possible.

Certainly don’t leave it in a 12” pot for a few years or DD will have to cope with dead tree as well

A tree in 12 inch pot probably isn't going to do to well, could you get a big half barrel type pot (might need to drill drainage holes) then should be alright for a good few years.

If you buried DD's pet a year ago then there's probably not that much body left (I'm guessing hamster size, bigger bones take longer to decompose) and will most likely all lift out together with roots anyway. I'd probably do it while DD's not around just incase, but if you think about the amount of small animals that live naturally in peoples gardens, all soil probably has decomposed little bodies.

Thanks, the dead pet thing doesn't bother me, it won't have decomposed yet, I had the same when I was small. I only mentioned as the tree means a lot to us because of it!, I do have to repot though? I'm so scared in going to kill it!
Ok, a barrel size thing sounds good, I don't have any ground I can put anything in so that's going to be on a search list for me now.
Thanks, I shall find a nice big pot DD likes and then I'm sure I'll be back needing it explained to me slowly....