The problem with car seats I would imagine is even though it is law to have one it could be seen as a luxury to own the car.
I doubt that's the reason - motorcycle helmets are VAT exempt as they're classed as protective equipment: www.gov.uk/government/publications/vat-notice-70123-protective-equipment/vat-notice-70123-protective-equipment#childrens-car-seats-and-travel-systems
And let's reiterate because this is coming up every other post:
VAT is not a tax on luxuries.
VAT is not a tax on luxuries.
VAT is not a tax on luxuries.
VAT is not a tax on luxuries.
VAT is not a tax on luxuries.
VAT is not a tax on luxuries.
VAT is not a tax on luxuries.
VAT is not a tax on luxuries.
VAT is not a tax on luxuries.
VAT is not a tax on luxuries.
Is that enough, do you think?