Thank you for your query,
Your child seems to have Acute Urticaria
of the skin which may be an allergic reaction
due to any change in diet, or any exposure to pollen or plants and some times due to internal allergens like worm infestations.
Check for any new food or any exposure to pollen or to pet animals to look for thecauses fo this allergic reaction and if you findy any avoid exposing them.
Hurting while walking is due to rash with itchy sensation which may be percieved by the children as pain.
You can apply calamine lotion externally over skin in the rash areas and oral antihistamine like cetirizine
can be given for 3 to 5 days depending on the rash in prescribed dose.
Consult your Pediatrician
for the examination and to look in to the causes of allergy.
Hope I have answered your query.