I took the leggings and attached them to the bottom half of dress that were no longer wearable by the girls. I sewed the leggings but stopped prior to attaching the waist elastic.
Then I took the dress and chopped it where I wanted to attach it to the leggings.
I cut a piece of elastic roughly the size of Darth Baby's waist because I wanted all 3 of the girls to be able to wear these. This turned out to be a mistake as I ended up having to sew two 1" tucks when they were done.
I placed the cut skirt (this one was lined and I kept the lining because it was sewn to the dress in places) and the legging right side out with the elastic right side facing the skirt. This was non-raveling elastic so I simply overlapped it.
Then I sewed it around 1/4 inch from the edge.
I flipped the elastic up and stitched 1/8 inch from the edge of the elastic on the skirt.
Here is the bear cub modeling hers. She took two seconds to flip around for me to take a photo and flipped back around to continue watching Peppa Pig the second the flash went off.
The nerdlet wouldn't even take them off for me to sew the waist smaller and even wore them to bed.
That is how long a size 80 (9-12 months?) is on my five-year-old. I really like that I'm no longer throwing out these dresses that are stained or aren't worth repairing so that they can continue to wear them.