Where can I find more cartoon videos like BBC's Muzzy for a young child to learn Mandarin?
I think you mean Muzzy? I've never seen that show, but I do have a 5 year old and I regularly let him watch videos from CCTV's official video site which has full length episodes of things like Thomas and Friends in Chinese Mandarin.

My recommendation is to show your child 小小智慧树 link to video page here. This is a popular children's TV show in China and is educational, directed at younger children and has quite simple dialogue. Each episode also has a simple song and poem which my son found easy to sing along with.

On the video page you will notice their are videos marked 《小小智慧树》 followed by a date e.g. 20120928. These are the full length episodes.

I find showing my son these shows are much more valuable than having him stare blankly at cartoons as they encourage participation via dance, song and activities.