We took DD when she was 2.7 I think and stayed on Tresco. She loved it - lots of beaches, being on a bike with DH, picnics and little walks, boat trips to other islands, lots of icecreams, hotel pool twice a day. It was a lovely holiday and we spent 8 nights there I think, in June a couple of years ago
The downsides were just the difficulties in getting there - even flying seemed to take ages, and if we did it again I'd get the sleeper from London then helicopter to the islands I think. And if the weather hadn't been good it would have been grim. The water is always cold though, at least 2 or 3 degrees colder than in Cornwall.
The other thing is that the boats that go from island to island are biggish, and the men who run them are really helpful, but watching people with buggies wobble up and down the steps to the boat and lift babies and buggies in and out made me really glad we only had DD and no buggy. Unless you are both really competent at getting into and out of boats with babies and buggies stuck under your arms I'd think twice about lots of excursions off the islands once you're there. You might be better off in Cornwall.
Re the Boden references, Tresco especially is full of Boden and Joules clothes, and people who like an aperitif before dinner (like me)