You need: a pen, scissors, Lil Leggings pattern, ruler, and paper for your new pattern (I used freezer paper)
Start by tracing the original Lil Leggings pattern.
I add about 1/2" around the sides and 1 1/2" at the waistband for a little more room. Part of this is to make it slightly less fitted than a legging, but part of it is because my daughter is in between sizes right now.
I eyeball about where her knees would hit in the pattern and make a little x on either side. This is where it will start to flare out, compared to being fitted all the way through.
Measure your child's inseam. Eden typically wears a 4T and her inseam is about 18". Draw a line 18" from the crotch of the pants.
I draw a dotted line straight down from the leggings pattern end. This helps me visualize where to flare it out. You could probably stop here for a full length leggings pattern.
On your hemline (the one 18" from the crotch), mark a spot 4" from each dotted line. This will be how much your pants flare and may need to be adjusted for larger or smaller sizes. Again, Eden wears a 4T.
I freehand a curve between each x. There is probably a technical way of doing this, but this is easiest and turns out fine.
For the waistband, I use the same length for the size recommendation, but I double the width. This makes it a cute fold-ever waistband, perfect for yoga pants. It also helps hold up the pants if your rib-knit isn't super stretchy (like if you are repurposing a shirt) and your little one is a string bean. Now you have your pattern and can construct the pants following the leggings instructions. If you don't own this pattern, you should! It is so easy and a great way to use up t-shirts! I can crank out a pair of leggings or yoga pants in about ten minutes now.
Here's my baby in her yoga pants getting ready for her first day of school:
We had a case of the sillies. :) [her backpack is the Made By Rae toddler backpack pattern- so worth it. I made one that I love for a fraction of the cost of a new packpack]