Years ago, full-figured gals had very few choices in workout attire including frumpy, elastic-waist sweatpants and over-sized t-shirts or men's "jogging suits" -- nylon sweatpants with a matching zip-up jacket.
Some people simply use sweatpants, others use cotton yoga pants or similar athletic wear, but really the best and most authentic way to outfit your trousers is the same as the top - purchasing a gi from an online source.
No matter how cute and sexy a costume is, if it's too tight, itchy, awkward, or otherwise uncomfortable, you may end up wishing you had gone as Suzy Sweatpants instead of Provocative Polly.
Walking into the place where you are going to fill out an application in sweatpants and a T-shirt probably won't land you the job, or make your application stand out at all.
You can use sweatpants or pajama bottoms for the pants and a plain white long-sleeved shirt if you can't find or make a peasant style blouse in your child's size.