Have you considered giving tokens; Boots, elder GS can always buy toiletries there or Smiths (magazines, Newspapers and some computer prepherals. Neither shop is licensed so it cannot be spent on alcohol.
Having said that, I would not give anything to someone who cannot be bothered to thank me for my gift and I would be quite prepared to point that out if it was noticed one GC was getting a gift and the other wasn't.
On a personal level, both GC are still at primary school. I would always give something to direct descendants, no matter what age, but a niece has just had her 18th birthday. I gave her a cheque considerably more than ther normal sum and said that from now on I will send cards but no more presents.
Army horses loose on London streets