Yes and No

Yes- You should go earlier in the year to spark interesst by demnstrating the instruments (or having your kids demonstrate).

No- You do not want them working on their own. I like to give as little information as possible on how to make an initial sound (just the basics) to see what naturally happens. The purpose of placing kids is not to teach the foundations of sound production, it is to see where natural abilities lie. I usually prep them before we start by stating that we all have physical characteristics that make us all unique. These physical characteristics make certain instruments easier and harder for different people. It is ok if you are not successful on any particular instrument. We will find one that is the best option together.
I sometimes also tell the child and the parent when making the decision that I want for band class to be the best part of their day. We want to avoid choosing an instrument where you have to work 2-3 times harder than others in the class to be successful.