Are there any other blockchains that can spawn child-chains within their own blockchain like NXT's Ardor platform?
With special transactions, Bitcoin has the capability of supporting sidechains. This is different from childchains in that the sidechain itself is a different blockchain and is not extended from the main blockchain. The paper describing sidechains can be found here: https://blockstream.com/technology/sidechains.pdf and an implementation of those sidechains can be found here: https://github.com/ElementsProject/elements.