Altering for large body type

I hope you guys can help me! I've been asked to sew a baptism dress for an 8 year old little girl, that is a size 14/16 youth. She is overweight and already developing, so the mom likes the style of the garden party dress to hide her buds. I made this dress in a size 10 for my daughter, so we tried the dress on her and it was tight. We couldn't even button the back closure. Even sizing up to size 12 might be snug on her so I was wondering if I could extend the length on the waistband piece to give her more wiggle room? Could I also cut the front/back bodice away from the fold by about 1/2inch to add a little more fabric/ease? I would also need to extend the yoke by slashing and spreading under the armpit. I'm afraid if I try to grade the pattern up to a size 14/16 that I will make a mistake. I only have a week to get this done. The parents have been frantically running around trying to find a dress to fit this poor little girl. I want to make her a dress she can feel beautiful in but I need a little direction on how to make it work! Thanks!