Unless you are going for several weeks, then definitely pick either Peru or Brazil, both countries have so much going for them.
If it’s just 2 weeks, I’d say Peru as distances are vast in Brazil ie if you fly to the Amazon, that’s a full day travel plus another day flying back. You’ll still need to fly in Peru but most flights will just be an hour unless you go north which most tourists don’t do.
Peru - with teens, I’d say 3 days in Sacred Valley , go white water rafting, do the via Ferrata and zip lines at Skylodge, spend a day riding or mountain biking in the hills, potter around Ollantaytambo, Pisac, see the ruins and markets, salt flats and Moray site.
This will also let you acclimatise easier to the altitude. Then train to Machu Picchu, maybe stay overnight and have a second visit as different circuits see different things.
Then Cuzco, lots to see, maybe go up one of the rainbow mountains , Arequipa if you like a city, drive to Colca Canyon across the desert, see the condors, amazing scenery, relax in the hot springs, lots of hiking, biking, riding.
Then there’s Paracas on the coast where you can fly over the Nazca lines , go sand dune buggies, see Ballestos islands with penguins, sea lions etc. its also a relatively short flight up to the Peruvian Amazon. It’s a very under rated destination but one of our best holidays.
Brazil is fab too, especially Pantanal for wildlife and Rio is interesting (despite the “dangerous” destination tag it has).
For both, I’d suggest a pre organised private tour , Peru in particular it’s not recommended to drive yourself. Far easier and cheap to get private drivers and guides.
Galapagos - don’t tack this on, far better done as a separate trip with Ecuador.