iXS Hammer Knee Guards
The iXS Hammer series offers great protection and is one of IXS' most affordable lines. They're strong and reasonably light at 300g for the pair. A combination of the Armadillo impact compression shell and NockOut™ impact absorbing padding gives the protection you’d expect with any good trail type armor. The open-back and AeroMesh™ material keep you well ventilated, cool and comfortable during a long ride or all day in the bike park.
I brought this for my daughter, i got xs and it fits her well, she says she's comfortable wearing it beacause it's lightweight and it got soft elastic straps.