Trust aside..
Going by your original plan, if I were you I'd give each of the grandchildren as minimum the threshold limit for their group plus the 3K limit small gift for the said year. Anything above those to go their parents (since you are paying same tax anyway). No point of wasting the tax free amount, money is hard to earn and easy to lose. It is better in your grandchildren's hand than the government's. Yes they might squander it away, at least that is their mistake to make but as minimum it benefits them. If their parents brought them up decent, they should be able to understand to spend wisely.
Maybe you can have a chat in advance with their parents and advise them of the plan you have in mind so they can guide the grandchildren or even put in an accompanying letter with your will to say so.
That would be the way I see it.