Hi pillowcase,
We are an English/Spanish bilingual family. DH and I are British so the language at home is English; we live in Spain and oldest 3 DC (DD1 8, DD2 6, DS 4)go to local Spanish school, so speak only Spanish there.
We felt it was important to teach the DC to read in English before they learned to read in Spanish as Spanish is a phonetic language and therefore much easier to read. This was made easier by the fact that they don't really teach them to read Spanish here until age 6. With DD1 this worked brilliantly. I bought the Oxford Reading Tree series and we tried to match where she would have been level wise if we had been in the UK. I found a link on another thread this week to a reading age test so have just tested her level now and she has a reading age of 10 years and 4 months (she'll be 9 in July), so I am pretty pleased about that. Her Spanish reading is behind her English but still average in her class. We are under pressure to encourage her to read more in Spanish at home, but I am happy to let her continue reading in English as she is now at a level where she does this for relaxation and I love that.
With DD2 we have been a bit, well, crapper for want of a better word. We have followed the same tack, but have been a bit complacent and have let her daily reading time slip to every day / twice a week etc. Having just done the reading test for her too (reading age of 6 yrs and 5 months; she will be 7 next month) I am re-motivated to keep it up. She is right in the middle of Spanish reading learning too now, so I wish I had pushed her a bit more earlier.
We are just starting out with DS - letter sounds etc.
As for writing, we have been more laid back about this. At one point I did try and get DD1 writing stories in English but with her Spanish homework etc it all seemed a bit much. We have decided instead to concentrate on her reading, figuring that she will, passively at least, pick up spelling and some idea about written language through that. She does sometimes write English stories off her own accord now.
Having said all that, we are planning to move them to the international school DH teaches at for secondary, so they will do IGCSEs there. As such, we don't have your issue of working towards a qualification at home. That does sound like a fairly mammoth task depending on homework and extra curricular activities.
If I were you (and I am no expert, just basing it on our experience!), I would start by concentrating on the reading in English. I think this is the key to accessing the written side of the language. If you can instill a love of reading (and are prepared for the cost of providing a small library in your DC's bedroom!) then you are setting them up brilliantly.