Sidechains are chains which allow 2-way peg, that is, moving bitcoins to the sidechain and then back to the main chain without trusting any third party, like you do, for example, when you move Bitcoins to say, Ripple (there you have to trust Bitstamp or another gateway). As such, there's no sidechain in existence, simply because the technology has not been implemented yet.
2-way peg is orthogonal to merged mining (you can do each of them separately or both). It is generally assumed that a sidechain would have merged mining because the bitcoin wizards designing sidechains accept that merged mining is generally desirable because of its superior security properties.
Compared to atomic swapping, 2 way peg is much slower but it doesn't require a counterpart that wants to move coins in the other direction.
So when sidechains are implemented, it is likely that most of the transfers are done with atomic swapping and only the first movements are done with 2 way peg, which is mostly there as a last resort in case you can't find someone else that wants to move the same quantity in the opposite direction.
Here are more details on how 2-way peg works:
How is a side-chain merging back to Bitcoin chain protected against double-spend? Where are the side-chain mining fees?