I have a lovely set from little green radicals. It's a good weight of cotton.
I recently bought a cute set from Dunelm but it's ridiculously thin. I don't see much toddler bedding in shops and obviously can't check the quality or thickness online.
It doesn't even need to be thick, but maybe on par with adult bedding in not feeling flimsy.
Toddler refuses to use a single duvet so we're limping along with his toddler duvet for the time being.
Edited to add that I'm talking about duvet covers. Toddler cotbed size.