You haven't mentioned what kind of school you plan on sending your daughters to - is it a public, private or international school? In an international school, tuition is commonly in English and many different curricula from around the world are taught in the various schools. The facilities in international schools are usually very good so I imagine there would be plenty of support available if your nine-year-old struggles a bit. Fees can be high though, so it would be ideal to negotiate an allowance for education in your relocation package. If this isn't possible, a private school could be a good option - less expensive but many private schools are bilingual with half the day taught in Spanish and the other half in English. Public school, on the other hand, is entirely in Spanish.
There are more details on our Education and Schools in Mexico page.
Best of luck.