This is a mild form of eczema seen in children.
It presents as irregular but round, scaly patches which are hypopigmented. It is more common in children with darker skin tones.
It is often found following sun exposure (as this makes the patches more noticeable) and is most commonly on the face but can also be seen on the neck and upper arms. It is usually asymptomatic but can be mildly itchy. In the cold weather it can be more dry and scaly.
Diagnosis is clinical but could be confirmed using a Wood lamp, mycology scrapings, or rarely a skin biopsy.
Treatment is not usually required if asymptomatic. An emollient can improve dryness, and a steroid can improve any itching or erythema. It will resolve after a few months and the colour will eventually go back to normal. Advice to families can be to avoid sun exposure.
References
https://dermnetnz.org/topics/pityriasis-alba/
https://www.bad.org.uk/ResourceListing.aspx?sitesectionid=159&itemid=7790