I was having a lot of troubles with youtube videos. Every video that the kids played taked cpu up to almost 100 %, so the video quality was very bad.
After a little research, i found that flash player in linux doesn't use hardware acceleration, so i tried two things: first, redirecting youtube to youtube html5 site, and then, using a dedicated player: minitube
Minitube works great, no problems at all, and the kids are very happy with it.
I really recommend this app, watching videos it's the first use of tech limited teachers, so a good performance in this area is crucial.
Juan