Welcome to the forum.
Wow - talk about being chucked in at the deep end! However, before deciding on how to structure your classes, I recommend meeting both of your prospective students for an initial get-together. You might find that "doesn't speak a word of English" actually equates to managing "Hello" and counting to ten or even more. I lost track of how many Spaniards told me they didn't speak a word but then managed a very basic conversation. The same goes for the child. In whose opinion is this child doing very well at English at school? Did that come from her English teacher? Is that teacher a native speaker?
My initial reaction is to say that it won't be a great idea to try to teach them together but you might not have a choice if you want the work.
The most important thing is to get a level test done with them both as soon as you can and go from there.