I know how hard this is for you Heartwrenched as we're in the same situation. We were sending cards and Christmas and for birthdays but have stopped now as I doubt they were being passed on.
We always bought 2, one to send and one for a memory box we have for them. I suggest you do that and in addition you could open a savings account for each in your name, and put money in their instead of buying gifts.
If you do get to see them in the future, you'll be able to draw that money out for them but if not, leave details of the accounts in your will to ensure that they'll get the money once you're no longer here.
IMO it would be unwise to involve the other GP's as doing so would put them in a potentially very difficult position.
I'm sorry but I think it unlikely the parents would be willing for any gifts, cards or letters you send to be given to the children. That would mean there's some contact with you and if they were willing for this to be the case, you'd have that contact now.
Unfortunately the GC in cases like these are the innocent victims; their wishes and rights not being taken into account.
Again, I'm sorry you're in this situation.