Hi E. ;-)
I have ten brothers and sisters.. and we all agreed that at age 18.. the little ones became big ones.. hahahha.. so were taken off of the "child gift" list. You mention that your children are 19, 6 and 1 and that they are still receiving gifts, so it seems that you and your family have not established what you all meant when you agreed to "just give gifts only to each other's children".
You can begin a conversation with your family using this as the opener and mention that you have been thinking about what it means to be part of this gift receiving group with respect to age. You can highlight that the legal age for adulthood is 18 and you think that it is appropriate now to include only children under age 18 in this group for receiving gifts from the family. Let them know that you are including yourself and your family in this, so it will immediately eliminate your 19 year old from receiving gifts as one of the children.
It probably will be accepted well since you mention you are not close to this family and only see them once or twice a year.
hugs,
A. R.N., Energy Medicine Practitioner