For the really little ones I think the term "ballet" is used very loosely. DD started dancing at age 22 months and they called the first half of her Talented Toddlers class "ballet" but it was really just about learning to point your toes, straighten your knees, and stand up straight while jumping around looking cute in a little pink skirt. She did not begin working on anything that I would consider real ballet until maybe age 6, when she was on team and was required to take two, 90 minute ballet classes a week with girls a year or two older than herself. Prior to that she knew the basic positions and the DT would make gentle corrections about feet and posture but that was about it.
I would be a bit concerned about a studio that allows jazz or modern without ballet, since ballet is truly the foundation of dance. DD's requirement was two (90 minute) classes when she was really little, and then the requirement went up to 3 classes weekly, but with a recommendation that they take 4. We did have some dancers that did hip hop only but for any other classwork ballet was required, even for rec dancers.