WE are using it with our 5th grader who is 11. My situtaion is a bit different but similar as well. We needed to start at the beginning as my son is an English Language Learner. I am AMAZED at what he is already accomplishing after just a few lessons in PLL. We do skip some things that I know will cause more frustration than help, and other things I will allow oral (when it specifies write). I plan to do one lesson a day for now, but may speed up. I just loved the lesson we did recently which covered “to, too, two” and that our son really got most of the answers correct. It was so affirming to me as his teacher. I am so hard on myself sometimes (aren’t we all) b/c I wonder if I am not the best teacher for him, but to see him fly through those and really get it. Wow!
I love how short the lessons are and he does too. He would not do well with something long and a worksheet. We’ve BTDT. I am using ILL with my 9YO son, a 4th grader, but I am planning to go back and use a few lessons from PLL with him as needed.
I am really pleased with PLL and ILL and the ease of use. They are great for an ELL student from my experience thus far, and I would think a child needing remediation would benefit for many of the same reasons my son is.
I personally wouldn’t fret over it at all. I am realizing my 7th grade son needs some remediation and I only wish I had realized it sooner. I am having a really hard time getting him to go back. I (and he) are frustrated but he needs to know basic sentence structure. 
Life is always fun and challenging! I’ll say that! Best to you on this and hoping it is going well!