Eczema (Atopic Dermatitis): This is a common condition that can cause a rash in the described areas, especially in families with a history of allergies or asthma. The rash can be itchy and may worsen with scratching.

Contact Dermatitis: This condition occurs when the skin comes into contact with an allergen or irritant, leading to a rash. It's possible that all family members were exposed to the same allergen or irritant.
Scabies: A contagious skin infestation caused by the mite Sarcoptes scabiei, which can cause a rash in the groin, thighs, and behind the knees. It's highly contagious and can spread quickly within a household.
Tinea Infections (Ringworm): Fungal infections that can cause a rash in the described areas. They are contagious and can be spread through skin-to-skin contact or by sharing personal items.

Syphilis: A sexually transmitted infection that can cause a rash in the groin area. Although less likely, it's crucial to consider this diagnosis, especially if the rash is accompanied by other symptoms like fever or lymphadenopathy.
Lyme Disease: A tick-borne illness that can cause a rash, although it typically starts as a single lesion (erythema migrans) and may not initially appear in the described areas. However, it's essential to consider this diagnosis, especially if the family has been outdoors in areas where ticks are common.

Pityriasis Rosea: A rare skin condition that can cause a rash on the trunk, thighs, and groin area. It's often preceded by a herald patch and can be accompanied by itching.
Keratosis Pilaris: A condition that causes small, rough patches on the skin, often on the thighs, arms, and buttocks. Although it's not typically itchy or painful, it can be uncomfortable and may be mistaken for other conditions.