About to give up (I can't find the right size)

I chose this dress to make for my daughter's second birthday in June. She is 19 inches(chest) and 19 3/4 (waist). According to her measurements she should be a size 12-18 so I made a muslin bodice and it felt too tight. OK, I made the next size up 18-24 and it fits perfectly...right now...but it doesn't look like it has any room to grow and she is still 4 months from her birthday. Then I made a size 2T and while it fits nicely around the belly it's too big around the shoulders and chest area. What do I do? I really like the dress. I think the instructions are great but unfortunately I don't have the time to fiddle with sizes and, what's most important, I don't think I'm skilled enough to do that. Would it work if I sew the bodice only with 1/4 inch seam vs 1/2 inch? I know it's not much but maybe it will do the trick? I'm to the point of finding another pattern. :(