It's likely stained from various soils and grasses. I'd use a facial scrubbing glove just on the knees during each bath, then dry, then add the bleaching cream, being careful not to get on clothing or linens until totally soaked/absorbed by the skin and disappeared to the touch.
If you have no scrub glove, use a 3M sponge with scrubber instead, but not rubbing until the skin is raw. It didn't happen overnight, and won't disappear overnight. It could take a few weeks to get it light enough to match other skin. If the child complains of knees being raw or tender, stop everything, including soap, on that area for 2 weeks until well, then begin again more gently. It will eventually lighten, but takes time. God bless and help you. : )
Alot of what you probably see as dirt is dead skin. The best method to get rid of this dirty "look" is simply apply baby oil or some other moisturizer to your child's knees or elbows after you have bathed him. This should help!
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