Bitcoin nodes voting
The actual stated development process can be found here. That said, there's become a rule where enough core developers need to "ACK" it like seen in this pull request.

Will there be a procedure for voting? If core devs feel it is safe, tested, useful, well written, match coding style, etc. then it gets merged into bitcoin. This is usually not easy and the bitcoin development archives and the numerous pull requests that haven't made any headway should make this obvious.

Will Sidechains get in? The big thing to note about sidechains is that at least a few of the core developers are actively working on it. This means they're going to try their hardest to get it in. So there's a better chance than most features people usually propose.

Do nodes vote? They can, of course, refuse to upgrade their bitcoin client to the new version that has sidechains, for example. That's entirely their choice. But actual getting into the bitcoind code, they do not vote on. People can and do, fork bitcoin and use that as their node (Luke-Jr and Eligius being the most prominent example)