How to indent sub-headings in Pages Table of Contents

Are you auto-generating your Table of Contents based on the Heading styles?

If so, when you create your Table of Contents, each of the different Headings are created with a common style.

You can then select any of the lines and use Inspector -> Format -> Text to set the style for those entries, using the same paragraph tools you're familiar with, including font, size, style, spacing and indents.

All headings of the same level will have the same style attached. It isn't possible to have two 'Heading 2' lines with different styles in the TOC. If you need that, you'll need to create an additional style in the document (e.g. Heading 2.2), which can have the same document settings as 'Heading 2', so it looks the same in the main document, but it can have a different style applied when incorporated into the TOC.

Have Show Ruler enabled on the View menu. In the ToC, click on the sub-headings you want to indent as in this image:

Then click and drag only the ▼ on the ruler to the right as far as you want the sub-headings indented.