This could be hives
which can occur in setting of diarrhoea( abdominal infection) and worm infestation.
To treat the same, taking syrup cetirizine
would help. Application of calamine
lotion would soothe the skin too. To treat worm infestation, a single dose of albendazole
might be required. Kindly consult a paediatrician for diarrhoea and worms.