Why do my two older cats eat my kitten's food?

About 2 weeks ago, we bought a new kitten and kept him in my room for a while (with food, water, and a litter box). Then we showed him downstairs and he goes down there everyday now. So then we moved his food, water, and litter box into the semi-big bathroom we have. Now my 2 older cats are entering the bathroom and eating the kitten's food and sometimes knocking his water over. The 2 cats get well fed enough, so I don't understand why they do this. How can I stop this from happening?