I read so much about bilingualism, but it all focuses on the “oral” part.
I have many questions about the written part: Is it better to learn one language in writing first and then the other? After how many years can you add the writing of the second/third language? Or can you do it all in parallel? Do you know whether there is any research on this topic?
Questions about when to introduce literacy (reading/writing) in one or more languages have a similar answer to questions about when to introduce spoken or sign languages: when the need arises. What matters is the need, not the timing.
Literacy typically serves schooling needs, and associated needs. School policies may specify preferences about introducing literacy simultaneously or sequentially, and children will accordingly adapt to this schooling need.
Policies are of course one thing, and what interests (little) human beings, as well as what we all are capable of achieving, is quite another. It is, for example, natural for a multilingual child who is being introduced to literacy in one language to start wondering about literacy in other languages. If this is the case for your children, just play along with whatever questions and puzzles they may have for you.
There is no need for you to either worry about “neglecting” one language whose printed form is not being taught to your child, or to delay introducing literacy in another, whose printed form may not be taught in school. Your children will become literate in the languages for which literacy is of use to them.
You can check out research on this topic in Professor Viv Edwards’ book, Learning to be Literate: Multilingual Perspectives.
Do feel free to contact me privately, if you wish to discuss these matters in greater detail.