For what it's worth, In the Marshmallow Test book, the author mentions that prior to the test, the experimenter spent 15 minutes interacting with the child to build trust. They gave the child a bell, and the researcher would explain that if the researcher left the room, the child could ding the bell to bring the researcher back. They'd do a few iterations of this, with the researcher promptly rushing back each time, in order to build trust.

Perhaps not a complete solution, but the problem was at least addressed.