Margaret O'Brien was going to be Alice in Alice in Wonderland, but the role was given to Kathryn Beaumont because Margaret O'Brien didn't realize that The Secret Garden, released in 1949, would turn out to be her last film for MGM. Nor did she realize that a bit player in that same film would ultimately provide the voice of Alice in Disney's Alice In Wonderland, even though Walt Disney had already publicly announced that Margaret would be his "Alice." Gladys predictably thought her daughter was worth much more than Disney was offering to pay. She must have figured she had Walt over a barrel, since he had already introduced Margaret to the world as the star of his "Alice In Wonderland."
"My mother had a big fight with Walt Disney," Margaret admitted in an interview years later with Allan Ellenberger for Classic Images. "What it was all about, I don’t know. I think it was over money. And he was going to sue us - it was a big deal." Walt didn't sue, he simply corrected his mistake. Four days after he announced that O'Brien was in... he announced she was out. Or, more politely stated, that Margaret O'Brien and her mother had withdrawn from the deal. Disney then turned to another young woman who had been also been under contract at Metro, and also recently let go... Kathryn Beaumont. A mere four hours after her test was completed, Disney offered her the role. Recording started June 19, 1949. One wonders, then, what to make of stories about the Disney talent search search, looking for the perfect voice that would be "not too British for an American audience yet English enough to satisfy the British."
Margaret O'Brien hardly fit this bill. Her English accent in The Secret Garden was rocky at best and widely criticized.
What seems more likely is that the "not too British" phrase was originally used to describe the wonderful quality of Kathryn Beaumont's voice... and only later promoted as what Disney had been seeking all along.
Given the fact that identifiable celebrity voices were chosen for "Alice" (i.e., Ed Wynn and Keenan Wynn [later replaced by Jerry Colonna]) and that it was the first Disney animated film to give voice credits, Margaret O'Brien may have been recruited for the star power she would bring to the marquee. For a time, the film was to be part live-action, part-animation, and again, Margaret O'Brien would make sense.
In Neal Gabler's 2006 biography of Disney, the author states that "[Walt] had floated the name of childhood actress Margaret O'Brien not only because O'Brien might attract an audience but also because she was under contract at MGM and her involvement might entice MGM to distribute the film rather than RKO, with whom the Disneys were disillusioned."
Today, it's almost as hard to imagine that Margaret O'Brien was first choice for Alice in Wonderland as it is to imagine that - had 20th Century Fox been willing to loan her to MGM - Shirley Temple would have been Dorothy in The Wizard of Oz.