I am a teacher (4th grade) and I appreciate you taking the time to introduce MDing to children.
Here are some strategies that might be helpful.
First, start with show and tell. Bring in enough interesting artifacts for each kid to hold one. Then during the rest of your talk time you can say "pass" every two minutes or so. This should hook them immediately and keep them interested (at least in what they are going to get next). Be very clear about who passes to who and how to handle certain objects. Don't hand out anything that can get damaged. (I lost a Chinese jade figure when a tenth grader dropped it "just to see if it would break")
Second, once their hands are occupied, bring out the cool tools. Detector, probes, diggers, sand scoops, headphones, etc... Briefly explain each item, but they aren't going to care about ground balance and the science of metal detection. It is easy for experts and enthusiasts to overtalk things and kill the fun.
Third, after a few rotations of your items, ask for some guesses about what some of the more unusual relics are. This gives the kids a chance to "discover" or figure something out. You may even choose two or three of the weirdest (items not kids) and say "I don't know what these are and I was hoping someone here would know"
Fourth, explain how your seeded hunt is going to work. I would use a sandy or woodchip area if at all possible. New diggers are brutal on the turf and most kids are going to just want to find something. Plant one item for each kid and let them keep what they find. The dollar store sells matchbox type cars cheaply, mix in some "gold" new dollar coins, and some foreign coins.
I have many more ideas if you, want but I realized I just slipped into teacher mode.
Good luck.
Fathead