Since your search is for for 3rd-grade Russian, a great "feature" would be a Quick-Check Answer Key designed specifically for busy parents or teachers.
You can find complete answer keys for this specific workbook on popular homework help sites like GDZ.ru or through curriculum resources on YaKlass . from Golub's workbook gdz po russkomu za 3 klass segodnia stranitsa golub.v.t
: Highlight words where 3rd graders often make mistakes on that specific page. Since your search is for for 3rd-grade Russian,
Instead of just a list of answers, this feature would include: gdz po russkomu za 3 klass segodnia stranitsa golub.v.t
: Next to the answer, include a tiny "reminder" box of the grammar rule being tested (e.g., "Suffix -ek/-ik rule: if the vowel drops, write -ek").