They already are learning that from the adults in their life. If they want to get beyond they need to do something different. There are two choices: experiment and see what works, or learn from others. The latter is much easier - IF you can find out what others have done. There are many modern farming methods that could apply well to the village garden, but there are subtle things that have to be got just right or it will be a failure just like the adults in the village warn you. The kids who can read have a much better chance trying the right experiment in the village garden and getting a good harvest, while those who seem to try the same thing without reading first are likely to make one of those small mistakes that ruin the harvest and fail the experiment.

you're right, learning an basic arithmetic would obviously be something to teach then but also with some practical skills (how school used to be for the west)