wow, first off kudos to you on getting your first gig and it seems to me that maybe you bit off even more than you can chew?
It sounded like you're only issue was the patter which is good because most people will come onto the forums and will simply start a thread about how thy're hired for a gig in a week, and they have never done one before, and what tricks should they do, and etc.
so to start off, i personally think that because of the large age difference you will have a particularly difficult time adapting you're procedure to fit.
but of course that is what you are asking for, so the one big peice of advice that i can give you is that you shouldnt underestimate the intelligence of a six year old, they dont want to feel that you are talking down to them, so to say, and neither do the older kids, so i would say that your best bet is simply to play it safe with tricks which concepts are easy to understand, not cheesy little kid ones, but still play big and impress the older ones, which would propably mean tying not to including a lot of pick a card material.
just as an afterthought, this may sound pretty self explanatory, which it is, but maybe if you listed the effects/routines you were planning on performing.................................................