Both are not on direct train lines so it will mean connecting trains from main line.
as a local, my POV is that they are both lovely but both very busy. St Ives streets are much narrower and feel more crowded whereas a lot more space in Newquay, but still busy.
Newquay does have stag and hen dos but it's nothing like the media has portrayed, irs improved a lot too.
I love not but if I was going on holiday I'd choose Newquay just because there's lots to do and not so hilly as St Ives 😅
I live near Newquay, it's pretty quiet in term time if that's an option. I love Newquay living nearby but I'm bias, it gets a bad reputation but everyone is local.
wheelchair would probably be easier to in Newquay compared to St Ives.
ST ives is more up market, Newquay is more of a typical tacky seaside spot.
Newquay has a zoo, several arcades, aquarium, boating lake, miniature railway and lots more! Several beaches, some easier to get to than others with someone with mobility issues like your gran, some of the steps are steep to the in town beaches.
st Ives is probably more historic.
my son is autistic too, he found St Ives really overwhelming as everyone was packed in the narrow streets but loves Newquay!
there used to be a bus that goes from Newquay to st Ives, an open top but I think it may have stopped.
the shops in St Ives are more independent and unique. Still unique shops in Newquay but has some more known shops too but not great.
newquay has supermarkets in and near the town on walking level compared to St Ives if you are self catering.
it sounds like I'm favouriting Newquay, it's because I live nearby and know more about it but have been to St Ives too. St Ives is definitely 'posher' whereas Newquay is pure tack!
I also love Looe if that's an option.
If you can I would consider term time, I know it's difficult with school but it will be considerably quieter for your son!