SESC Pompeia and SESC Santo André usually have a cool lineup of children's theater with prices way more affordable than traditional theaters. Since you guys live in the south zone, Santo André is pretty far away, but Pompeia in the north zone makes up for it with accessible ticket prices and plays that are usually designed for different age groups. It's worth checking out their website to see what's playing that month.
Teatro Infantil Gibiteca is also an interesting option, and there's a lot of activity from small theaters in the south zone itself - Vila Madalena has some nice little venues that usually have more creative shows. Really, your best bet is to check the city's programming or sites like Timeoutsp or Guia Cultura because it changes a lot depending on the season. Get the schedule from at least two or three places and compare them: the play, the time, the ticket price, and whether it makes sense for the kids' age.