Tan - I have used BG on both kids - they are fine.
GK are a bit too wide for Miss M - she's a scrawny thing - but they fit DS beautifully.
Where they sit depends on the build of ya kids ya know? My nappies always sit under their belly button - they are really long in the body. I always found Magicalls to be quite a big sizing - the large was a bit too big for DS even now (but he has no hips or butt to keep it full I guess!)