
Well I finished it and S has been living in the costume since then. I think it was a hit. She is a little bummed that the top is not actually a bra but a shirt (she is 4 and it is Fall in the north country so there was no way that was happening anyway). I made the shirt adjustable with ties in the back so it could go over a jacket or be let out as she grows.
I used the same fabric as I did for the fin except in a blurple (blue purple) color, and I can now say I will never use this fabric again. I hatted it. It was hard to sew without puckering and pulling the fabric, couldn't be ironed without basically melting (even on my lowest setting), and is only dry cleanable. Luckily I only spent 2.50 on it, so if I ever have to replace this fabric I can without worrying about how expensive it was.
Materials: 1 yard of fabric for the top, I used the shinny blurple I described above, but bathing suite fabric would be ideal for this or something else that has a little stretch (I guess I'm not that smart).
4 yards of ribbon (or bias binding). I used a sheer teal color. Matching thread, pins, paper and a pencil.
So how did I do it.
Step 1: Measuring
- Measure around your child's chest (not too tight or anything) we'll call this #W
- Measure from how high on their chest you want it to how low (i.e. how tall the top will be). Here if you want to to be more of a crop to you could) we'll call this #H.
- I also measured from the center of her chest to under her arm (in a line parallel to the floor not sloped) so I would know how far I had to slop the pattern down to the back part.
- Measure from the top of your child's chest H measurement over the shoulder to the center of her back about half way down (where the top of the shirt will come in the back), we'll call this #S.
Step 2: Making a pattern.

I decided to take some large paper and make a pattern for this step. I took my measurements and a pencil and went to work. I added 2" to all (except the under arm) measurements to allow for a 1" seam allowance (because of my horrible fraying and tricky to work with fabric) if you want a more traditional 5/8ths seam allowance then add about 1 1/4" to your measurement.
The pattern will be placed on the fold of your fabric so you will only be creating half the top (and therefore it will be symmetrical once cut.)
-To get the height of the patter measure up one edge of your paper H+seam allowance.
-Next measure on the adjacent edge (W/2)+seam allowance.
-From the far edge of the mark you just made draw a straight line up (H/2)+seam allowance.
-Now from the top mark (the first mark you made) and the top of your line you just made make a sloppy ski jump type line.
-Now is when you need the measurement you took to under your child's arm. you need to make sure you slopped quick enough but not too quick.
(Once you look at the picture this seems so much less confusing than actually explaining it. It is actually quite easy.)
Step 3: Cutting fabric.

Now cut out your pattern (you can hold it on your child to see if it will be about right just remember to hold it in the center of their chest).
Place the pattern with the H side on the fold of your fabric. Cut one piece.
Repeat so you have 2 of the same pieces (if you wanted you could do this in 2 different color fabrics.) Your top is going to be 100% reversible.
Cut your ribbon.
- 2 pieces at S+seam allowance.
- 4 pieces at about 8" (for tying the back together so give or take what you think you will need).
Step 4: Pinning

With right side of one of the shirt piece facing you pin one shoulder strap into place. I wanted about 4" between the straps so I measured from the fold line to the side 2" and placed the strap there and pined it. Line up the strap so it is even with the edge of your fabric and laying down the front of the shirt. Repeat on the other side.

With the same strap find the spot on the back you would like the strap (the closer together the better for keeping on the should but not close enough that it will be in your seam allowance (I did 1 1/2") and pin it there (it is tricky to find what way the strap should face so it goes smoothly over the shoulder without twisting but take the time to figure it out cause I didn't and had to use the ol' seam ripper). Repeat on the other side.
Next take 2 tie straps and place them on one of the sides and pin..
Now take your second top piece and lay it (with right sides together) over top the other piece and pin around the top, side (with the ties) and the bottom of the top. You will leave the one side open for turning.

NOTE: I added some extra ribbon on the bottom of the top you can too if you want. It just added a little more to it. Just pin it in when you are pinning all the ribbon for the straps and ties but keep it out of your seam allowance on either side. I used some liquid stitch on it so it wouldn't fray the ends.
Stitch around the 3 sides. and turn.

Now fold the seam allowance in on the raw edge and insert your ties and pin.
Top stitch around the entire piece.

I had to tie the ribbons so they wouldn't fray. And you are done!
Hope you and your little one enjoy.
Here is as back view. I had to take some extra ribbon to tie the should straps together in the back cause they were a little long and kept sliding off. If I were doing this again I may cross the straps over to the opposite side. But live and learn.