@Thoman Ideally should take your entire solution across as a whole if that's how you have organized your customisations and a version of the entire solution has already been installed into your Production environment?
You could, however, potentially create a separate solution or patch solution to contain just your Child Flows and export that from your source environment as managed and import it into your target Production environment. But you would be running the risk of deployment failures due to dependency issues if your Child Flows have introduced new dependencies on components that you don't include in the new solution/patch solution and these don't already exist in your target Production environment.
Perhaps a cleaner or more long term approach should be to reorganise all your customisations in your source (development) environment into smaller solutions, each container fewer components. This will enable you to have more granular control over which components you deploy, and think more carefully about component-interdependencies, sequence of deploying solutions etc. Once you have successfully deployed the newly reorganised smaller managed solutions across all your environments, and these cover all your original components, you can then safely delete the previous larger solution from them.