The bus is the only and thus the best option. However, be prepared for the relatively long travel time.
Traveling from Krabi to Kanchanaburi involves a 6-hour bus ride. This is the only available option, so there aren't many decisions to make. However, it's worth considering the variable prices ranging from 450 to 900 THB depending on the provider.
The bus is also the cheapest option, with prices starting at 450 THB. It's advisable to check current prices with the providers.
The bus is suitable for families, but keep in mind the long 6-hour duration.