Hello Ivy,
Welcome to Atlassian Community!
When it comes to customization, currently, there is no option to change the name of the system buttons in Jira or Confluence.
Regarding the name, "Add a child issue" appears only in next-gen projects. On classic ones, it will show "Create subtask".
There is no feature suggesting this change, so please, feel free to raise one on jira.atlassian.com (project JSWCLOUD) adding more details about why this feature is important for your environment.
Regards,
Angélica
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