Jennifer Nelson organised the visit to Aston Hall, a Jacobean mansion in Birmingham, but failed to count the number of children when they gathered for the afternoon trip.
The key stage 1 teacher at Manor Park primary marked the register while pupils were having their lunch to save time, and allowed children to play in the playground before they gathered for the trip.
Eileen Dowd, head of the Birmingham school, said she thought initially that the two children had been left intentionally, but soon realised that no arrangements had been made for them.
When Miss Nelson returned to school she admitted she had not counted the children and that it was not until their journey back that she had a "gut feeling" a child might have been left behind.
She was sacked last year from the school where she had worked for four years and is now employed as a supply teacher.
England's General Teaching Council found her guilty of unacceptable professional behaviour after hearing she had put the health and safety of her pupils at risk. It was told that she had already received a verbal warning after taking a group of seven-year-olds on a trip to a greengrocer's even though they did not have permission to leave school.
Miss Nelson told the GTC hearing: "I admit that I was wrong and that I should have checked the children were present," she said. "Although I am not making excuses, I ask you to take into account that I was under considerable stress, especially as my teaching assistant had been off sick."