Legalities (UK) of arranging horse/horse care days etc

eml who owned a riding school is the expert here. But you need a Local authority licence and you need insurance - the two depend on each other.
The staff or people working with young people - even as volunteers, need security clearance too.
Plus some first aid provision and training.
And if the kids are there for a day, premises allowing for loos, hand washing etc.
There are two UK organisation involved in encouraging and recognising RSs - the BHS and the British Association of Riding Schools. They probably have lots of info.
It really depends on who you are and what you intend to do.
Because in practise some pony owner give access to horses to kids on an informal basis. My inner city grand children had their first rides from a woman who brought a pony to a local field in the summer after school and the mothers paid £1 for the kids to be led round the field on the pony - There are charities that provide access to horses for children and if that is the sort of thing you have in mind, I would look to see what you have locally and whether you could participate.
 
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