Tailed beasts are inside the body, which is why babies are usually chosen (because by when they're old enough, their body is already 'used to' the tailed beasts' chakra). However, I'm pretty sure that just because the tailed beast is in someone's body, that doesn't particulary means their chakra can be used as the jinchuuriki desires.

... Because simply, the chakra is the tailed beast. They're an enormous mass of chakra, it's not their chakra which is sealed within a jinchuuriki, it's the bijuu themselves. Lol.