Youtube isn't the place you'd really want your young students browsing, though as we all know there is some great stuff there for kids, and that's why Totlol is such a great idea.
It's new video sharing site and community which allows parents and teachers to set up their own channels and share videos for ages 6 months to 6 years and above ( Found quite a few that I liked). The videos are collected from Youtube, but are shared through the Totlol community site so there's no danger of your students clicking through to something unsuitable.
The site is still in beta, but it looks like there's some nice content there already, so this would be a good place to start looking or get your younger students in the habit of looking for good, safe, educational content. You can also sign up for your own free account and start contributing towards the community.
Well hope you find this useful and hope you enjoy this alien song!