I have been a regular shopper at Old Navy since my oldest son was born. I love their selection and the quality of their clothing. Their prices are also reasonable, especially when you hit their sales.
I found these Canvas Cargo pants for my youngest son and only paid about $10 for them. You can get these pants as small as 6 months and as big as 5T. When you buy them in the baby sizes, they have an elastic waist; however, the bigger sizes feature an adjustable waist system. I find this feature to be a lifesaver because all of my kids are very thin! They button close and have a zip up fly.
These pants come in a variety of colors and are 100% Cotton. I will say that they do shrink a bit after washing them the first time. These pants are a quality piece of clothing. I would not expect anything less from Old Navy!