Practice, I know it sounds tacky but it helps

Write up what to say, it does not have to be word for word but at least bullet points

Know the information so you can help every one

Speak to the person in the back row

Talk slow

Keep yourself open to help do not run a group

Think through all the logistics

Have others to help with discipline

Always have a backup plan 

Do not read you speech try to say it

Use the correct tone for the age of the kids

Do not talk down to them

Treat kids like they are older but expect them to act their age

Practice with your helpers so that both of you trust each other