When I was a child we were not permitted to use a gift until a thank you was written. It was considered to be disrespectful of the other person. So was being late but that is a whole other story. I taught my children to do the same.
Accepting a lack of manners on some else's part is enabling and is the reason it becomes more prominent. It would be the last time they got a gift from me, especially a gift that took hours of time and effort to produce. If ever asked about it I would politely mention the reason for no more presents. I would accept that I wasn't going to get thank yous from them but I wouldn't continue to give gifts that weren't acknowledged at least by phone or email. If you allow people to disrespect you they will continue to do so. Ann in TN