RTW Alteration?

P is tall (90% for height). I have bought her a few sundresses for summer but in order to get them long enough I have to get her 6 or 6x. They are all too big around the chest, a lot too big. I have never done any alterations to a RTW garment, so before I start hacking at dresses I paid good money for, I'm looking for suggestions for the proper way to alter the dresses and potentially any other dresses I may purchase in the future. I'm assuming I just need to take in the side seam. Do I unpick the seam that is already there? Is there a way to alter and leave the extra seam allowance so I could let it out in the future but won't make it bulky to wear for now (in case her growth spurt slows down). Thanks. Jane