We used to live in Sao Paulo (not San, that's Spanish, not Paolo, that's Italian) when I was a teenager. Mum is Brazilian, Dad is English, we'd lived in Brazil before so obv that all made it easier. I visited recently with DP (English, speaks some Portuguese) and our DD, who was 7 months at the time.
There's lots that's fab about it and in some ways I'm actually pretty tempted to relocate there myself (we're aiming to spend a couple of years in Brazil when DD is about 5 or 7 or so). The cultural life is excellent, and the food is, as well. Traffic & driving conditions are awful but the metro is pretty good. Brazilians are very good with kids both culturally and organisationally: you will get preferential treatment in metro and airport queues because of having little kids.
It is however a big, dangerous, polluted city. At the moment Brazil is pretty expensive for English visitors so you need to be sure if the financial package concerned. There are good schools but they're expensive too; if your eldest is only three it probably wouldn't be a big impact but likewise health costs are (I believe) also potentially high.
I would certainly recommend Brazil in general as a place to live for a couple of years; Sao Paulo wont be easy but there are so many people of all sorts there, and so much to do, that it will have a lit of compensations. Unless it's all a bit too culture shock-y - I take it you do live in London / are used to big city life?