In a sidechain you can't create new coins (not even with block mining reward), only transform coins from the main chain. In a 2-way pegged altcoin (like Ethereum or any other) you can.
But it doesn't make a lot of sense to send coins to a sidechain when you are going to have inflation while price is pegged to bitcoin. Holding this sidechain coin will give you less profit than holding bitcoin itself.