Most high street shops go down to a size 6 (topshop and miss selfridge go down to a size 4!) Next sizes are quite big tho, i'm usually an 8 and wear a 6 from there. All of the other usual ones go to a 6 too.... river island, zara, new look etc.... i even think primark does now.
Next womens is too big yes, but kids is fine as again there are no hips in the trousers.
baytrading used to be fab until they closed down but i have found another similar called INTERNACIONALE...they do size 6 and i bought a littlemix style playsuit in black n white which is a brilliant fit. no baggy armholes and straight up and down cut.the clothes there seem to be smaller thn average too-thats been my favourite haunt for the past year