I can understand where you are coming from but I wouldn't get too crazy about this if the grandparents don't see them that often.
And when I say "that often" I'm talking like how often I see my granddaughter who lives out of state so it's like
three/four times a year.
I go overboard but with things and it's probably more for me than her because I see so little of her.
As the twins get older, and I'm assuming they are little since people are feeding them, get the grandparents to
buy toys if they want to indulge. Craft projects are good for a lot more special time with the grandkids.
As for the size of the bites, you are probably right. Smaller bites are probably better but most toddlers want satisfaction NOW and if they don't get it, they let the world know, and you two are dealing with two kids.