My inclination is to say it means they were beaten/whacked on the butt with bamboos. It is a little bit strange though, usually we use "bogged" with "down" and say something like "I can't go with you I'm too bogged down by work" (like how you might get stuck trying to walk through a "swampy" bog--I put swampy in "" because it's actually technically different than a bog) So it's strange trying to figure out why Orwell chose the word "bogged" I'm not too sure.

I'm guessing bogged in this context is similar to being flogged, or being beaten physically. Bamboo might literally mean bamboo stalks, but it might also just mean any kind of stick. They probably used the phrase "bogged with bamboos" to sound more interesting (look up alliteration, it's used often in writing).