Newbie Gear Question

BOOTS, At the very least get a cheap pair,They will save your ankles,feet and shins!!! Goggles are very important too,As for riding outfits-They do have pads in all the right places,hip,knee, etc

For trail riding, here is my suggestion, somewhat in order:

-Helmet
-Boots
-Chest Protector (if you expect to be falling)
-Goggles
-Knee Pads (if you expect to be falling)

Other, not necessary, but nice to have:
-Riding pants - more protective material than jeans, leather and heat protection panels/material like around the knees for gripping the bike and by the hot exhaust, padding and rubber logos for protection, cool comfortable liner, made to stretch/bend, fit over knee pads, etc.
-Riding gloves - help avoid blisters, also have some padding to protect your fingers/hands
-Jersey, or at least a long sleeved shirt to somewhat protect your arms from trees/brush/crashes

Based on my experience I'd put elbow guards higher on the list than knee guards. Most of the kids seem to have more problem and tears over their forearm/elbow than knees when falling. As far as knee protection I prefer quality braces for myself and my kid over guards because the boots tend to put additional loads on the joint. Asterisk has both the Cell for adults and the Germ for kids. You will also want to look at some bark busters for your bike to help with hand protection and broken levers.