And the Academy Award winner, who plays a nanny with magical powers in the new children's film, has vivid memories of one particular babysitter who stole her sweets when she was recovering from an operation.
The actress was so distraught she ran away from home.
Thompson says, "I just had my tonsils out when I was seven and this girl called Paula, who was looking after us, stole my box of sweets. I was so angry I ran away. I can't believe she ate my sweeties. I still feel quite bitter about it.
"I took my sister, dressed up in our balaclavas, made some sandwiches and went round the corner to sit behind a tree next to our nursery school.
"We ate the sandwiches then went home and Paula opened the door. We said, 'We ran away but we're back now.' She let us in but never told anyone about it."