Here is a quick guide on how you can modify your rainbow twirl pattern to have faux shirring on the bodice.
* Grab yourself some elastic 9mm-15mm wide.. whatever you have handy!
1. Cut out bodice pieces according to the pattern. Cut an extra set of bodice pieces to be used for the lining.
2. Sew down one side seam of the bodice and bodice lining.
3. Position straps according to the placement in the pattern, pin them on the bodice and lay the bodice lining on top. Right sides of bodice and bodice lining are facing. Sew along the top.
4. Fold the bodice lining into position, press and top stitch.
5. Run rows of stitching around the bodice and bodice lining approx elastic width + 5mm apart.
6. Attach bodice to the skirt according to the pattern.
7. Measure the chest of your little one or use the chest measurements in the pattern as a guide. Cut lengths of elastic around chest measurement less 25mm-40mm (based how stretchy the elastic is that you are using, the more stretch the elastic has, the larger the amount to take off).
8. Complete the dress according to the pattern.
Note: To minimise the elastic bulk, I would probably overlock each side seam separately, then stitch the side seam. and top stitch the seam allowance down on either side of the seam, at least for the bodice section of the dress.
The same technique can be applied to the shirred dress pattern, but you just need to change the construction sequence so that only one side seam is sewn at the beginning of the pattern.