Hemming unfinished children's pants to allow for growth

I just bought my son's Cub Scout uniform pants, and they came unfinished. The old ones I thought would last 5 years he outgrew in two. (They have the elastic buttonhole tape to adjust the size)

The new ones are about 10 inches too long. He has 3 more years in this uniform and is expected to grow 5-10 inches in height over the next 3 years so I don't want to cut much off. What's the best way to temporarily shorten these to allow for growth without the hem being too "weird"?