Our child speaks native Finnish (appropriate to her age). We are moving abroad. Supposing the child learns to read and write in another language that uses essentially the same alphabet (probably some Germanic language; English, Danish or German, perhaps), will she also be able to read and write Finnish as a side effect, or with little extra instruction and practice?
Finnish is written and spoken in almost the same way, in that letters and pronounced sounds have an almost one-to-one correspondence. You can pronounce Finnish almost correctly by simply pronouncing each letter in order.
The issue of reading and writing in the formal "kirjakieli" versus the spoken dialect we use is a separate issue for a separate question.