Let them take the tax as planned so it covers their liability, then reclaim it back from HMRC on a R40 claim form (link below). You may have to satisfy the revenue that the bonds are entirely for the benefit of the Grandkids by signing a delaration or providing a undertaking to that effect, if they require it.
http://www.direct.gov.uk/en/MoneyTaxAndBenefits/PensionsAndRetirement
/MoneyInRetirement/DG_10024687