There is also waist band elastic that has draw string in it already. It is sold by the yard in most fabric store to do jog pant waistlines or similar. I have taken in lots of slacks by taking a little tuck in the waistband with the sewing machine. If the pants have front pockets, I sew through the waistband with a 1/2 inch pleat to the inside. The extra fullness under the waistband goes into the pocket and isn't noticeable. When the child grows, you just clip out the little waistband pleat.