It's OK to pre-heat your oven. It's a good idea, in fact.
It's OK to use the oven as an impromptu hiding place for things you don't want little hands to clutch or touch. I mean, who hasn't done this? It's done.
But if one person uses the oven to hide a large Tupperware cake pan full of iced sugar cookies from the kids before she leaves for work in the morning, and some other person pre-heats the oven to 450* Fahrenheit to make dinner in the evening, without checking the oven first, there will be smoke. And flame. Pools of flaming plastic will puddle up on the bottom of the oven. It helps if you had the foresight to place the cookie tub on the broiler pan to contain the plastic, and it's nice to have an 11-year-old to ask, "Should the oven be smoking like that?"
The cookies will be surprisingly well-preserved as long as you don't hit them with the fire extinguisher, but you should probably throw them out anyway, and the garlic bread will come out just fine on the grill.