my $0.02
First I confess bias: I only know the Canon line well. Nikon makes some of the finest cameras and lenses, but I only know them from my reading. Can't really help with the D90 details, but I know many users love it.
I CAN talk about the line of entry DSLRs from Canon; I've had an XTi, and an XSi (loved them, but now I moved on to bigger camera bodies). The current model is the T1i, and it has superior performance to the above and shoots video to boot. I think the most important difference between the Canon and Nikon offerings for your uses, is that the low light performance of the T1i is better (this is very useful for shooting in dim cages without using a flash that might disturb the animals, and for a bunch of other reasons).
I think you can pick up a lightly used T1i for around $700 without a lens. An XSi is less.
Canon cameras seem to have more menu-driven controls; Nikon seems to use more button-driven controls. That's neither here, nor there.
I can also tell you the lens line from Canon is more extensive and less costly than Nikon's. The lenses you choose will have a big bearing on the look and quality of you pictures. This is the real strength of the Canon system, if you ask me: better lenses.
I really like a macro lens for animal shots; you may want something different for family settings.
Whatever you choose, have a great time learning your DSLR.
You may find these helpful:
http://www.fredmiranda.com/
http://www.photozone.de/Reviews/overview
http://www.cameralabs.com/reviews/digital_camera_and_lens_reviews.shtml