1. Son and Father cross taking 2 minutes;

2. (a) Son returns taking 1 minute, OR
(b) Father returns taking 2 minutes;

3. Grandpa and Toddler cross taking 10 minutes;

4. (a) Father returns taking 2 minutes, OR
(b) Son returns taking 1 minute;

5. Son and Father cross again taking 2 minutes.

17 minutes.