Where can I watch Zettai Karen Children, or old anime in general?
Some legal and popular anime streaming sites are:


Crunchyroll
Manga Entertainment
Funimation
Daisuki
The Anime Network


The anime you're looking for, Zettai Karen Children, can be found on The Anime Network, and the spin-off series The Unlimited - Hyōbu Kyōsuke can be found on Crunchyroll.

As for the manga(s), Amazon is a good place to start.

Here's a list of legal anime streaming/download sites.