Once they get off the plane, they will follow the "purple" transfer signs to Immigration. They will go through Immigration and then will emerge into the baggage claim area. If they need it, there are luggage carts, usually off to the right in the baggage area.

Your parents will reclaim their bags, walk a very short way to go through the Customs check and then walk a short distance to drop off their bags to be rechecked. The bags will already have the tags on them for BOS that were put on at GRU. If they are using one of the luggage carts mentioned above, the baggage checkers, usually, tell you to just roll the whole thing to them and they'll take care of it.

As kents mentioned, there is a TSA checkpoint after the baggage recheck. Tell your parents to put their shoes directly on the conveyer belt, not in a bin. When I was there last week, they were very insistent about that.

After they clear the security check, they take the escalator up to the C/D gate areas. If they are leaving out of the "A" gate area, they will have to take one of the "moon buggies" over to that terminal.