Too Much, Too Soon?

For the past couple of months I've been letting Tabytha be more independent when it comes to breakfasts and lunches. She can get her own cereal, bowl and pour the milk. She's even toasted frozen waffles for herself and John. Tabytha can make herself peanut butter and jelly sandwiches or get a hot dog and heat it up in the microwave. No problems. She's recently started making her own instant oatmeal in the microwave with the tiniest bit of assistance from me with the milk and measuring cup.
Well. Today she wanted to make a grilled cheese sandwich. I said "OK". She got the bread, butter and cheese out of the fridge. Put the sandwich together and asked if she could put it in the toaster oven. I, again, said "OK". She's used the toaster oven before so I didn't think to stay in the kitchen and oversee her.
My mistake. About a minute after she puts the sandwich in the toaster oven she comes out of the kitchen and says the plates on fire. WHAT?!?!?!?!? WHAT PLATE??? I hurry into the kitchen and see that she put the paper plate she assembled her sandwich on IN the toaster oven and the heating element has now gotten hot enough and caught the paper plate on fire. I quickly turn off and unplug the toaster oven, go to the sink and fill a cup with water and open the toaster oven door and throw the water onto the blazing paper plate. In the madness of the paper plate fire I yell at Tabytha and quickly regret it. She's 7 years old. I'm suppose to be the 'responsible' adult and should have stayed in the kitchen overseeing her use the toaster oven properly. I should have done a better job instructing her on what to do and how to use the toaster oven to cook a grilled cheese sandwich, which she'd never done before.
After everything is under control I apologize to Tabytha for the way I spoke to her and yelling at her. I tell her if she'd like to try again another day I'd let her prep the sandwich, but I'd go ahead and cook it for her.
So. After all this, Tabytha has no sandwich. I tell her she can go ahead and make herself a peanut butter and fluff sandwich if she wants and that was fine with her. She made her sandwich, nothing caught on fire and she got to enjoy her lunch. (I may need a new toaster oven after this)