Hiking pants.

Pants for hiking. A little elastic. Retain wind and moisture (moisture a little retain, of course, specially sit in the wet is not intended as rain pants for example)
Protection material on the knees and buttocks.
Adjustable waist with elastic.
Built-in rubber to adjust the length of the pants, grow with the child, so for several seasons definitely deres.
Pockets in front.
Pa hanging bag where you can put a bottle for example.