What kind of weird little insect is this?
This is probably a nymphal stage of a stink bug (aka shield bug).

Based on the roundness of your specimens, I'd suspect that they represent possibly the 1st or 2nd instar development stage. 

I am unsure of the species currently, though I suspect this is an image of the adult (wrongfully labeled as a "bed bug" at its source):



 Source: Shutterstock 

I will dig into the Brazilian fauna to see if I can get you a species...