There is a growing trend of buying beet pulp to feed to livestock. I don't really understand this trend. Beets are extremely easy to grow either inside or outside. They are truly almost bombproof, do well in partial sun/shade.
Beets do well in a sunny window inside the home. I get a cardboard box- the cases that Powerade/soups come on work really well- put the box in a trashbag (the 13 gallon inside bags work well) then toss a whole bunch of potting soil (about a gallon) onto the bag/cardboard flat. Plant seeds, water, put in a sunny window and wait. In about a month you will be able to start harvesting your beets for either yourself or your animals. I prefer this method over rubbermaid containers due to the flats being fairly shallow and wide/long. The plastic containers would work well too. Just make sure to water sparingly.
Even easier would be to plant a raised bed/container right next to your animals pen. Then practice the pull and place method- pull up a plant or two and place into your animals cage/pen. This is what I do with Mangel beets. Mangel beets can be grown inside, but they get so large that I prefer to grow them outside. They should be planted in an enclosure as deer/rabbits/etc LOVE mangels, at least they do mine. Don't feed mangels till after they've gone through a good frost as it makes them sweeter.
I feed 1 reg beet per rabbit per day or 1 mangel beet (after frosts) per communal pen per day. Baby bunnies don't get beets till they are at least a month old, and even then I don't add the popples to the rations list. So they will only get one or two bites of beet per day- not enough to upset their tummies.