Ok I know this is rather specific, but who knows? There may be someone out there with perfect advice
Background: Swiss German is technically a dialect, rather than a separate language, though it is different enough from High German (or just, German) that Germans don't understand it, usually. Swiss German is spoken-only; as soon as Swiss German kids learn to read/write, they must by necessity also learn High German, because that is the language they will be reading and writing.
To understand the scope of the differences, e.g. Swiss German knows only two tenses, present and past perfect, whereas German has the full contingent. It is probably nearly as different from German as Dutch is.
DS is 5. We speak Swiss German at home, probably about 80% of the time (otherwise English). His Swiss German is pretty age appropriate, though he has started to translate English phrases verbatim which doesn't tend to work out.
For a year now he has attended German Saturday school and his High German (or 'proper' German) has improved massively, but only receptive. He now understands High German pretty well, all his bedtime stories and audiobooks are in High German and he is fine with them. He hasn't however yet pronounced a correct High German sentence, though recently he has started trying. He is now in the next year up at Saturday school and they are teaching the kids to read. But it is very slow going (understandably, with only 90 minutes/week). So far (two weeks in) they have learned the letter 'ä' (admittedly the first week was mostly for settling in.)
He has just started Y1 at school and reads really well (for his age) in English (Turquoise book band, which poses no difficulty). We feel he is secure enough with that for us to focus on his other language for a bit now (sideways extension, so to speak)
So: Does anyone know of any resources/strategies/experiences for teaching a Swiss kid who hasn't got frequent exposure to High German (as Swiss kids in Switzerland would have), to read and write in German? Online, workbooks, readers, ...? What kind of 'first readers' books/series could you recommend?
Alternatively, resources for learning to read and write in German in general? For young children. (Generally resources are aimed at slightly older children, as 5yos don't tend to learn to read and write yet in German speaking countries.) Any such resources we may have to adapt, as they would probably assume that the child speaks German (rather than Swiss German), but they would be a place to start.
Would be extremely grateful (and a bit surprised) if someone could recommend the perfect resources for us. Alternatively, any thoughts and comments will be appreciated :)