They do not forget you, but as they grow up, they have have so much else to take up their time, than g.parents. Normal and proper way of things. If we think back, how many of us sent as much as a burthday card to our g.parents???
Once my g.children go into double figures I stop sending them actual pressies, and send them money. Not a lot, with eight of them and me on nothing more than my pension, but always money - that is what they want. I love it when they (or their parents) let me know that are saving up for something and that pressie is going towards that.
Once they pass their 18th birthday, I stop those birthday presents, but still give some money for christmas. Always paid directly into their bank accounts, so I know they have received it whether or not they actually get round to thanking me.
I do not expect cards from them for birthdays, etc. indeed, really do not want them to waste their limited income on cards and postage. My pleasure in them is seeing them grow up, develop into lovely adults, leading happy, useful lives. As my family do not have children until around or into their thirties, I will not see any great g.children - which suits me just fine.
My grand children (IMO) are the responsibility of their parents, so any monies I may have in excess are likely to go to them (my children), rather than g.children.