My 4 yr old son suffers from itching, rashes and redness after every 7-10 days behind his knees & back of his thighs, hips and elbow joints?
Eczema. Eczema is a skin problem which causes dry, itchy skin, especially in the areas you mentioned. It can come and go for years. Bathe with plain water, only mild soap as needed, no scrubbing. Pat dry and apply a good moisturizer (no fragrance, thick, not a watery lotion) regularly. If it worsens and causes raw skin, see your doctor.
Atopic dermatitis. Atopic dermatitis is an itch that rashes (as opposed to a rash that itches) - it's the itching that comes first, and scratching it that causes the rash. The itchiest areas are typically the areas you listed, although other areas can be involved. Most kids outgrow it, although an unfortunate minority do not. Treatment depends on location and severity. A dermatologist can help you with that.