We visited Shetland a few years back to try and work out if we wanted to move further away from the big city (we were then - and still are - happily smallholding in the Borders, with easy commute to Edinburgh) but decided that moving to an island community was not for us. I know it sounds harsh - but you will always be an outsider as will your children. We get this even in the Borders, the other kids in my daughters' high school are all born in the Borders,and the number of cousins in the school is ibcredible... Not to say that we don't love being in the Borders, but we will never be "Borderers".... (But then I am not even British, so that has recently become more "visible").
With regard to livestock - as long as you can make your own hay or haylage you can feed goats, sheep and cattle quite well, but buying stuff in is extremely expensive. So if you lok for a holding/croft check out the land it has with it, soil quality varies a lot. You will also struggle to get new bloodlines or males for mating, esp for dairy goats. As to selling produce, there are probably quite a few other corfters doing that, so you would need to scout out the market. I don't think it is easy to make a profit.
That said, if you want to move for the "getting away from it all" life style - can't get fresher air anywhere else I think - and you have an alternative income to keep you covered for the basics, then I would definitely investigate further. But I wouldn't go without a definite - and permanent - job offer and loads of research. The islands are also quite different communities compared to the Mainland, and unless you live on the mainland close to Lerwick, your children will have to board at the High School during the week - another main reason for us not to move.
Shetland winters are long, dark and wet.... and very few trees.
Poly crubs are very expensive, Keder greenhouses are just as good and a lot cheaper.
I am not putting you off, but that's where we got too after quite a bit of research.