We swear by Children's Place for our skinny girl. The pants seem to be cut slimmer than most other brands so like Trish said, they don't have to be gathered as much when you adjust the waist. Plus, a lot of the styles come with 1/2 to 1 inch hem, so they can be let out if needed. However, I think their shirts run short (or shrink in length), so we always go a size bigger in the top.
Oh... and yes, Grace has outgrown a pair of pants in a single day! So much for the myth that they only grow at night :)