The OVA is only four episodes (http://en.wikipedia.org/wiki/Pet_Shop_of_Horrors#Episodes) -- they're reasonably faithful adaptations of some of the individual arcs from the manga, but you really don't get the whole sweep of the interactions with Leon (and his little brother) and D, and how they're all changed by each other.

As for child-suitability -- well, it is a horror series, and there are some fairly scary, gory moments in there, particularly in the first two episodes. I suspect it might be too confusing and too scary for a small child. Older kids who aren't phased by violence or scary creatures would probably be OK with it.