....And in a class that has 20 pupils from 20 different tribes?

Sasa shule za Nai and other cosmopolitan towns n cities itakua aje?

They can make it a subject, the same way they teach French, and even take it all the way to University.

lakini hii ya class 1-3 in Okuyu, Jeng' etc adds no value. private schools (where every parent who can afford) take their kids will continue teaching their classes in English, offcourse parents with Kids in public schools will feel cheated etc and complain, then Kaimenyi will force private schools to teach in mother tounge, etc.. etc..

Ok, what exactly is the benefit?

With laptops coming "soon", how would they translate

keyboard -
CPU
Ram
etc? I agree with @Obi, they can make it a subject for the study of the ethnic history, art, science and culture of the local dialect.

This age group, 5-9 ish, is the BEST time to create a multi-lingual child! Their minds are like sponges and don't question anything. I am of the opposite view. Kindergartens should only teach in English and ALL other languages banned. Re-introduce Swahili as mandatory and 1 local language as optional in Std. 2, once the child has learned to express his / her feelings and opinions properly in 1 language.
This mother-tongue thingy worked for the prior generation as kids were ALREADY 7-8 yrs old prior to school so one needed a bridging language. No longer necessary on a national scale.