Does anyone know what Dreambox will do if I tell Hanni to guess her way through? Will it figure out that she can't do it, and dial back the difficulty? (This is a regular game, not a pre-test game.)
Yes, it should dial back the difficulty in that case. What you don't want to do is let her quit out of the game and try something else because she can't get through it. My DD did that for a while and she basically got "locked," with nothing but half-finished games that were "too hard." We ended up taking a long break from Dreambox at that point and coming back later.
