It might be easiest to get the solicitor to draft a letter for the parents to sign. Otherwise though a court would probably accept any clear and unambiguous statement from the parents, e.g:
"To the court considering the application of Martha Smith for a Special Guardianship Order, in respect of the child Matilda Bloggs, to be made by the said court under the provisions of Section 14A of the Adoption and Children Act 2002:
We, the undersigned, confirm that we are the birth parents of Matilda Bloggs and that we currently have parental responsibility for her welfare.
We further confirm that Martha Smith is Matilda Blogg's grandmother and that we support the application being made by the said Martha Smith for a Special Guardianship Order for the child Matilda Bloggs.
Signed: Fred Bloggs / Winifred Jones"
https://www.familylives.org.uk/advice/your-family/fostering-adoption-kinshipcare/special-guardianship-orders/