No flights direct from UK to Forteleza?

Well, there's no reason (IMO the "flavor of Brazil" that you've come all that way to see is much more interesting elsewhere) to stay in S.P. any longer than it takes to recover from your international flight in and get out again to Rio on the next plane to Rio. (or maybe even comfortable overnight sleeper bus, if your kid will indeed sleep on one) You can check out the Japanese neighborhood Liberdade, the MASP museum (mostly Brazilian art), the Pinacoteca modern art museum, Trianon park, and enjoy strolling in the upscale Jardins neighborhood, looking at shops. There is a good bookstore, with items for kids. The ice cream in S.P. is the best in the world (Italian immigrant skill meets tropical fruit.)
Rio is MUCH more interesting (as I described in your other thread).
Salvador is 1.5 hrs from Rio by plane.
Or you could go by bus from Rio back down the coast to Ilha Grande (by bus and boat) and then to Paraty. Onwards from Paraty back to S.P. is 6 hours by bus (maybe a bit long for your little one).
Have a good trip.

Ah, and bring a folding stroller with the largest wheels you can manage; sidewalks are in awful condition. (Few Brazilians use strollers.)