The youngest of the Cromwell children, Sophie, states that she followed her siblings onto the bus to Halloweentown.
Marnie: How did you get here?
Sophie: I followed you onto the bus
Dylan: But you were asleep
If that's the case, how come her grandmother did not see her get on the bus if she used the front door? Or if she used the back door how come Marnie and Dylan didn't see her get on?