Immersion is the only way to go, kids are language sponges and learn quickly. The younger the better.
And the English learned will be proper English.
I'm with you Mike. From personal experience, when my wife and 8 year old daughter came here From Chile (legally) in the 70's, the only English the girl knew was "hamburger." She was put in a regular class with no ESL stuff. It was sink or swim. Half way through the school year she was fluent in English. At the end of the year, she had no accent. (Except for the word "yellow.") And this kid was no Mensa candidate. She flunked SPANISH in high school. Yeah, that's what I said.