There is no requirement for a grandparent to register with Ofsted.
They should declare to HMRC any money you give them, but may be able to have it written off as reasonable expenses. HMRC may determine they are employed, self-employed or neither, in which case the money is treated as an 'additional income' for whatever way in which they are currently assessed for tax. An accountant or certified tax advisor will be helpful in getting this clarified initially.
If the grandparents are still building up qualifying years of National Insurance payments towards the state pension, they should apply for Specified Adult Childcare Credits.
You might also consider using an Ofsted registered childcare provider for those reasons already stated by other contributors.
Whether you use grandparents or a registered provider, make sure you are very clear about expectations, boundaries, rules, etc. before commencing any arrangement. Very few people do this: they often just assume their way of bringing up children is the only way and it doesn’t occur to them that another person might do things differently. This can lead to awkward situations, disagreements or even quite acrimonious falling out, which is worse if family is involved. (In a sense, this is even more important when family members are providing the childcare, since there are no written guidelines, regulations, policies, contracts, etc. to serve as a point of reference.) So think to plan ahead, rather than running into trouble.