I mentor a group of first gen college freshmen who are ESL. Although they were born in the US, they only speak Spanish at home. It is not apparent in their conversational English, but woefully apparent in their writing skills and ability to read anything complex or nuanced, and they’re struggling to complete college level work.
These students did not grow up reading for pleasure - only did assigned reading, which is not enough, and as a result, their vocabulary is very limited. I suggest that your children read all types of English literature, not only to develop their vocabulary and comprehension, but to learn idioms and literary references. At least a book a week, and more during school breaks. Pick an author and read everything, then go on to another author from the same time period or genre, so they develop a deep understanding.
I really believe this is all they need to do at their age. Tutoring won’t help.