They are often very open to such events, especially if it's a slow news day. Sending advance notices is a MUST for this. Generally at least three weeks is what is required. Make sure to include contact info. Keep the actual notice brief and to the point.
Prizes are great to have on hand.
I like to be sure that I have something to send each child home with. Little goodies can often be acquired from professional associations who's work is in some way related to the type of activity you are conducting. Although budgets keep getting tighter for government agencies, hand-outs can generally be acquired from such places as NASA and various environmental agencies. Allow plenty of time for these to make it through the mail. Your attitude will be contageous.