Remove limit of 50 in child_items query

What does this MR do?

With https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29165 merged, we can remove the limit of 50 items without going over the complexity.

It now defaults to 100.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content