Thank you for everyone's responses.
Our family's native language has very simple grammar and does not utilize verb tenses at all. As is often the case whenever the parent's native language is not English, the main language spoken in the house is not English, and so our boys do make lots of verb tense errors. This is to the point that their online writing instructor has noted it to me and asked for remediation. That is why I got anxious and hoped to find a solution.
Although I understand much of grammar can be learned from immersion, I want explicit practice in the major areas -- verb tenses being one (jolly there's 16 verb tenses in English!). It has been over 15 years since I learned English, and I can generally get by in normal sitch. But, in more academic spaces like literature writing, I realized I am often lost whenever dealing with when and which verb tenses to use.
I will look into Warriner and Rod & Staff. As for Latin, it seems like a whole can of worms. Should we really learn a whole other language just for the sake of grammar? Of course, I know Latin has many other benefits, but we are already focused on proficiency in our native language at home and also plan to take on Spanish soon. On that note, will Spanish similarly provide better understanding of verb tenses as like Latin?
Thanks again for all the ideas and discussions!