Upvote:2
Anything is possible, but trouble at immigration strikes me as unlikely. Foreign grandparents of US-citizen grandchildren visit the US on B visas all the time. That they are traveling with the US-citizen grandchild would not have any logical bearing on their tendency or inclination to overstay or otherwise violate the conditions of their visitor status.
Whether the child's parents are US citizens, permanent residents, or (long-term) nonimmigrants should have no bearing on this.