Add id to GraphQL nodes in epic_children.query.graphql

What does this MR do and why?

This MR fixes a ESLint GraphQL failure introduced in !74528 (merged).

Screenshots or screen recordings

This MR doesn’t introduce user-facing changes. It fixes a failed pipeline in the master branch.

How to set up and validate locally

eslint --cache --max-warnings 0 --report-unused-disable-directives --ext .js,.vue,.graphql . --parser-options=schema:tmp/tests/graphql/gitlab_schema_apollo.graphql

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.