The design test - is the article designed for young children?: Riding hats
You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
Riding helmets are generally of a standard shape and construction for both children and adults and sizes up to 6¾ (jockey skulls up to size 1) may be zero-rated irrespective of how they are held out for sale. However, although children’s riding hats may be less sophisticated in terms of design and appearance than adults’ in the same size, they can still be zero-rated over size 6¾ provided they are specifically fitted, adapted or appropriate only for young children. All helmets held out for sale for adults are standard-rated.