Replace custom SVG icons in Twenty Twenty-One

That’s not how that file works, and including it will just load the class definition, it won’t create a Twenty_Twenty_One_Child_SVG_Icons object or override Twenty_Twenty_One_SVG_Icons. It’s also not how child themes work. Child themes let you override templates specifically, not arbitrary files and classes.

Instead, use the twenty_twenty_one_svg_icons_{$group} filters to modify the array in your child theme. The filter is the best option. E.g. twenty_twenty_one_svg_icons_social. You will also need to filter twenty_twenty_one_social_icons_map so it detects new social network URLs you add.