How safe is Sao Paulo for my two sons, 6 and 3 years old?

My Ex-Wife is living in Sao Paulo, Once she has a job, apartment, appliances, etc.. she wants our sons to come and visit her for a month in the Summer and a month in the Winter. I have full custody of our kids but I want them to have a relationship with their mom.

How safe is Sao Paulo for two young American boys? A few people I know from Brazil have told me about being robbed at knife point on buses and on the street, also you read about horror stories of tourist or people with ties to america being kidnapped and held for ransom, is this a common occurence in Sao Paulo? If my Ex got mad or angry and I couldn't reach her would ther be any help in locating and getting my sons back? (local police or social workers)

What questions should i ask me Ex about where she is living, etc... to find out if it is in a safe part of town? She will be working so what should i ask her about daycare or who will be watching my sons while she is away?

As you can tell I'm very hesitant with this idea and would much rather have her visit them here, till they are older. Any feedback from anyone who has raised kids in Sao Paulo would be appreciated, thanks.