I can't say enough good things about the SciShow Kids YouTube channel. My four year old calls Jessie (the presenter), her "science teacher", and routinely surprises me with information she has learned there. They're a great length (usually 3-5 minutes), so you can 'tune' the time.

https://www.youtube.com/user/scishowkids

As far as PBS shows go, Nature Cat is a favorite (and well written enough to be enjoyable for parents as well).