Rail Passes and such

We are flying into London in late March for a couple of days and then going onto Paris for 10 days. I wanted to find out if there are any special packages or plans that I should consider for Rail travel. I have never taking the train from London to Paris and want to share the experience with my wife and children but also want to take advantage of any packages that would be useful when in France to see some of the other cities via rail for adventures. Any suggestions or comments would be greatly appreciated.

Thank you