This Spy Kids 2 - The Missing Numbers (elementary) Mini-Unit is perfect to practice missing numbers skills. Your elementary grade students will love this Spy Kids 2 - The Missing Numbers (elementary) Mini-Unit. All of the numerals are missing from Betty Bat's cave... and there's a lot of letters in their places. What happened? A fun introduction to Roman numerals, also featuring reading comprehension, writing prompts, and MUCH more!