A cotton-blend with a bit of added stretch, often cotton mixed with a small percentage of elastane, tends to be the most durable practical option for kids pants, offering both flexibility for movement and enough structure to resist thinning and tearing at the knees.
Denser, tighter-woven fabrics generally outlast looser knits for anything involving a lot of ground contact, like crawling, kneeling or sliding into home base at a game. A thin, loosely knit legging fabric can develop holes faster under this kind of repeated stress.
Reinforced knees, either through double-layer fabric or simply a tighter weave in that specific area, are worth looking for if your child is particularly hard on pants, since knees are almost always where kids' pants fail first regardless of the base fabric.
Darker colours also tend to show wear less than pale ones, which is a practical bonus rather than a durability factor, but it does mean a pair of pants can look presentable for longer even as the fabric itself gradually wears in.