The channel ID is the same as the user ID. Here’s some v5 endpoints:
https://api.twitch.tv/kraken/users/<user ID>
https://api.twitch.tv/kraken/channels/<channel ID>
These are also available:
List of user objects by comma-separated logins
https://api.twitch.tv/kraken/users?login=<user login>,<user login>,<user login>
List of user objects by comma-separated IDs
https://api.twitch.tv/kraken/users?id=<user ID>,<user ID>,<user ID>
List of channel objects by comma-separated IDs
https://api.twitch.tv/kraken/channels?id=<channel ID>,<channel ID>,<channel ID>