These are lovely gifts.
I am sure you did not expect anything 'in return' but a 'Thank You' is a natural response and the least you should expect. When you give a gift you also reach out to the recipient showing your love, respect and appreciation. Not thanking you is like refusing an outstretched hand. It probably feels like it too.
My nephew and niece behaved just like yours. Finally, I decided to bring Christmas presents to a family shelter instead. I was sitting on the floor helping a little one with a toy when all of a sudden sticky little arms almost choked me and I got a huge wet kiss.
Being grateful is not learned, it's natural and it does nurture human interactions both ways. It has nothing to do with 'payment' or 'returns'.