I'm doing a sewing project for my niece's birthday. She'll be five. She's fairly average and this is for an elastic waistband so it doesn't need to be precise. I want it to be a surprise so I don't want to ask for measurements. Thanks.
If she's wearing a size 6, her waist measurement is about 22". Also, if you leave the waistband casing open for an inch or two, you can make a last-minute adjustment if it's needed, then quickly whipstitch it closed... I've done that on gifts before.