Go for it Witzend!
You've done it before and it was a success, and why should the smallest child be cheated of a treat the older ones have had last year?
You sound as if it is right up your street and even if it is tiring, well worth it to see the little ones' glee.
It seems most men fall into the "can't be bothered" attitude the older they get, and completely forget that what is "tat" to an adult can well be the most marvellous thing in the world to a child.
I still hang a plastic angel on my Christmas tree every year, not because it is anything startling - white plastic little child cherub - but when I was four and was given it, I was convinced it wasn't an angel but a true representation of Baby Jesus. Now I can see it for what it is, a cheap decoration, probably from Woolworth's, but in a corner of my heart it is still Baby Jesus. Something similar may just be the case with what your grandchildren buy on your shopping trip. I hope so.