Advise on how to keep English Tutoring engaging for my two students

So, I'm not an ESL teacher per se so please forgive me if this isn't allowed. I am just a native English speaker from America who currently tutors two children in English online for free as a favor for a friend of mine who lives overseas in South Asia. The girls are very advanced in their English since in their country, they start learning English pretty young. They can hold a conversation with me, write in English, and understand me when I speak with some limitations. But they still struggle with the more advanced grammar and as well with remembering to use their past/present/future tenses while speaking sometimes. I tutor each girl for an hour a week on separate days.

I am a STAHM with a limited budget but I buy children's books on Kindle to have them read out loud, and will help them go over grammar they are learning currently in their English class, helping them to prepare for upcoming tests. Our tutoring sessions usually are as follows: a structured English conversation on whatever topic interests them (i.e upcoming holidays, exciting events, interesting movies they've watched), a writing prompt to have them practice writing in English (usually a very short essay), and then they read a chapter of a book I have bought them on kindle for reading practice, reading out loud off of my screen. If they have exams coming up, I will review the content with them. We do our lessons virtually and utilize screen sharing for going over written material I obtain online.

What else can I do to make the tutoring sessions more engaging to two young children? Are there any workbooks you recommend I get for helping them with their grammar?