Because Heidi has been brought up in Switzerland, she would speak Swiss German (Schweizerdeutsch). The little boy would speak the dialect of German spoken in Frankfurt (click here for more information).
Four languages are spoken in Switzerland. These are Swiss German, French, Italian and Romansch. There is only a very small number of Romansch speakers (less than 1% of the population). Most of these live in the canton of Grisons (Graubünden) where Heidi's story is set.
The map on the left shows the French area (lilac); the Swiss German area (pink); the Italian area (green); and the Romansch area (yellow).
In the video below, Roger Federer, the well-known tennis player, is speaking Swiss German.