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Try the Lands End catalog. Good quality, reasonable price, and they take back everything! 
Boys Climber Cargo Pants, $25.50 Item # 17834-3A59. They describe them as: Tougher than roughhousers!
•Double knees
stand up to wear and tear
•Double-stitched seams defy rough play!
•Stress points are bartacked for durability
•Tots 2T, 3T, 4T
•Little Boy 4, 5, 6, 7
•Boy even sizes 8-20
These are similar to what my boys had, but with elastic in the bottom legs.
Boys Climber Cargo Pants, $25.50 Item # 17834-3A59. They describe them as: Tougher than roughhousers!
•Double knees
stand up to wear and tear•Double-stitched seams defy rough play!
•Stress points are bartacked for durability
•Tots 2T, 3T, 4T
•Little Boy 4, 5, 6, 7
•Boy even sizes 8-20
These are similar to what my boys had, but with elastic in the bottom legs.