We do not teach "dokąd" in the current version of the course. It seems that it was simply forgotten, or maybe the person responsible for the related skills was just too much used to the colloquial use of 'gdzie'. You are right, "dokąd" would definitely be a lot better. It is of course accepted, but it cannot be the default answer. This is one of the biggest mistakes made in this course, in my opinion - and it will for sure be fixed in Tree 2.0.
A colloquialism indeed... however, I remember having a conversation with a native speaker once. She asked me 'gdzie idziemy?' and I pointed with my finger in a direction saying 'dotąd'. She corrected me and said that I should say 'tam'. A discussion followed, but then she finally agreed, saying that 'dotąd' is acceptable after all. After this I was under the strong impression that saying dokąd/dotąd instead of gdzie/tam in casual speech sounds unnatural to some speakers, since it's perceived as too formal, but that might also have been an isolated incident that does not represent the majority of native speakers. Or maybe both forms just shouldn't be mixed in consecutive sentences?
Well, "gdzie" is very common in this meaning, so even in Tree 2.0 it will definitely be accepted, even if not default anymore. We are rather harsh with grammar, but after all, sometimes the reality is that something is just too popular to disregard.
Frankly, I don't think that 'dotąd' suits the context that you mentioned. It seems logical, but it doesn't work this way, in my opinion. Moreover, it's more often used to denote time:
'dotąd' = 'up to this time', 'so far', 'hitherto'. "Dotąd nie posiadałem samochodu, ale wczoraj kupiłem sobie Opla" = I hadn't owned a car so far, but yesterday I bought an Opel for myself"
'dotąd' as a border in space: (talking to someone while pointing at the map) "Pojedziemy stąd (pointing at Berlin) dotąd (pointing at Düsseldorf)". From here to there.
I remember that when I said 'dotąd', I was pointing to a landmark visible from where we were standing, located about a mile away. I guess that was the reason we agreed 'dotąd' was acceptable, since I happened to specifiy the distance.
Anyway, thank you for your explanation. I had previously thought that 'dotąd' had a broader range of application.
Seems that "gdzie idzemy" should have been accepted with a typo: it's "idziemy". It denotes going somewhere on foot, although often it also means that 'how' you are going somewhere is not that important.
For example it seems a lot more natural to say "idziemy do kina" to say "we're going to the cinema", even if we're going there by bus or by car.