i have searched some points, following may do some sense:
"Sidechains" are independent blockchains that typically have a kind of "pegging mechanism", where at least one of the chains (mainchain and sidechain) is "aware" of the other chain and both tokens are pegged 1:1.
"Child Chains", like the Ardor implementation, are much more tightly integrated into the main chain. In Ardor, all transactions of the latest 1440 blocks in all child chains are validated by all nodes from the main chain. That is a key difference to sidechains, where main chain nodes do not validate the sidechain (there may be clients that validate both chains, though, but it isn't mandatory).