In a 3 1/2- or 4-quart slow cooker, combine cream cheese, undrained tomatoes, milk, cumin, and fajita seasoning. Stir in chicken, American cheese, Monterey Jack cheese, and kidney beans.
Cover and cook on low-heat setting for 2 1/2 to 3 hours. Serve immediately or keep covered on warm or low-heat setting for up to 2 hours. Just before serving, stir in cilantro. Serve dip with toasted pita wedges and/or tortilla chips.