"I can't selfishly take journeys anymore because I have to take a little boy along with me," she told reporters Tuesday at a Los Angeles press junket for the film, out Oct. 4.
Before taking on the role, the actress, 49, told producers she was in – if they made it "an amazing experience" for her son Louis, 3½.
"I said ... make it so I'm not somewhere not paying attention because I'm so worried about 'Where is he? Is he having fun? Is this a good life experience?' " she said.
In the end, it turns out, everyone on set went above and beyond for her little one.
"[Producer] David [Heyman] turned a backlot of a sound stage in rainy London into a wonderland for a 1½-year-old," Bullock said. "Everything was bumper guarded! People go, 'What is that?' And I go, 'That is to protect a child's head! All of it!' "
While it sounds like Louis had a blast on set, Bullock – whose character has lost a child – was especially grateful to have her tiny tot at her side at the end of long days of shooting.
"I kept having to say, 'What would I do [if I lost a child]?' and I realized I might be far worse off than she is," said the actress. "You just have to go there and know at the end of the day, you can unplug and go home and do something a lot of ... people who were parents can't."