Re: Ballet shoes
Ballet shoes are typically 1 1/2 - 2 sizes larger than regular shoes which means you need to choose a sz. smaller than your normal. They should fit pretty snug BUT when buying them for a child I wouldn't go quite as small as I would for an adult ... you need to allow a little room to grow ... so 1 1/2 sizes should be adequate.
This is not a standard rule though and each brand may be a little different. I would suggest finding the shoes you plan to order and then do a search on that brand's sizing chart just to make sure.