Toddler's fracture or stress fracture: Although the question specifies "without trauma," toddlers often don't report minor falls or injuries, and their bones are prone to stress fractures due to their active nature and bone development stage.

Plantar fasciitis: Though less common in toddlers, it can occur, especially if the child has been walking more than usual or has an abnormal gait.
Sever's disease (calcaneal apophysitis): A common cause of heel pain in children, usually during periods of rapid growth.
Ingrown toenail: Can cause significant pain and is common in toddlers due to improper nail trimming or tight footwear.
Foreign body: Toddlers often put things in their mouths and sometimes in their shoes, which can cause pain if a foreign object is lodged in the foot.

Osteomyelitis or septic arthritis: Infections of the bone or joint can present with pain and may not have a clear history of trauma. These conditions require prompt diagnosis and treatment to prevent long-term damage.
Neuroblastoma or other bone tumors: Though rare, these can cause foot pain and are critical to diagnose early for effective treatment.
Child abuse: Unfortunately, non-accidental injury is a possibility and must be considered, especially if the pain is unexplained or if there are other suspicious injuries.

Tarsal coalition: An abnormal connection between two of the bones in the foot, which can cause pain during walking.
Kohler disease (navicular bone avascular necrosis): A rare condition causing pain in the midfoot area due to avascular necrosis of the navicular bone.
Freiberg's disease (avascular necrosis of the metatarsal head): Another rare condition causing pain in the forefoot due to avascular necrosis of a metatarsal bone head.