Since my daughter was growing out of her clothes so fast, I decided to buy her this Hello Kitty themed mini capsule wardrobe in the size that was two sizes bigger than her actual age. (She was 4 years old at the time and I bought them in the size intended for 6 year-olds.) Turned out, they were okay in the length but way too big in the waist.
I COULD have ripped all the seams and made the waist smaller by doing it in the right way, but I was pretty sure that she was growing up so fast that I would have had to adjust the size again before too long. So, I decided to do quick adjustments that were easily reversible.
These are quick and dirty ways of fixing the waist band, and they're very basic. If you already sew at all, you probably already know them. So, you can skip to the part 2 of this series, which might still interest you since I thought I was pretty smart when I came up with the idea. (You still might already know it, and in that case, I was the last one to know the method already existed.)
Pictured below is the start of my daughter's fall 2019 wardrobe. Maybe her (and my) obsession with Hello Kitty is obvious.