Is 6x like the bigger version of size 6? My daughter is so super skinny that I doubt she will ever be an X of anything? She is in size 3t-5T bottoms but 5-6 shirts because she is like 43 inches but only 37 lbs at almost 5 year olds. Right now she is pjs in size 5 but I would consider getting her a pair in size 6, but not sure if these would fall off of her.