Hi Juli,
there is no official carers ID card, although some local authorities do issue one. Some folk show their Carers Allowance letter, though I suspect you don't receive his Carers Allowance (and not everyone is able to claim this anyway.) You could take a copy of your Grandson's DLA/PIP award letter and say this is proof he is disabled and he requires a carer at all times and I'm his carer today.
Melly1