An expanding child form is not necessarily read-only. Check the child form's field permissions before sharing it. To not show any details about linked records (i.e. just their title), simply turn off Allow users to expand (or edit) linked records.
If you want to make the record clickable:
Enable the option to Allow users to expand (or edit) linked records.
Go to the child form settings (not the parent field) and make all fields read-only. The screenshot below shows the older editor’s Bulk Actions menu. In the current Builder, the same Make all read-only action is under Actions in the Form outline.
When all the fields are set as read-only, the 'submit' button will also be hidden.
💡Tip
This function also makes the records “expandable” in the gallery view, allowing users to "learn more". For example, you can make the record (card) preview some high-level information by default. But when the user clicks, it pops up to show full details.