Hi Rabbi Simon.
We are going away for two nights next week (not over Shabbos) and leaving the kids with a non-Jewish babysitter (she works at a Jewish day-care facility, so she’s relatively well versed in the workings of cooked food in a kosher kitchen). I am leaving the kids and her fully cooked food and I want to ask you if she can turn on the oven to warm it up. She can microwave it but it tastes better when it’s warmed up in the oven. She will not be turning on the stove. Thank you!
If the food is already cooked, she may heat it up in the oven. The reason for this is that the prohibition of food cooked by a gentile does not apply to food which is already cooked and only requires re-heating. The Sages did not extend the restriction to such a case.
However I should note that it is not ideal to leave the kitchen in the care of a non-Jewish child-minder while you are out of town. To mitigate the problem you should entrust a neighbour with a key to your home and tell your babysitter of this fact, ie that a friend may drop in at any time. In this way she will be inhibited from taking any licence with the contents of your kitchen (food and utensils) which could compromise its Halakhic integrity.
Enjoy your time away. What a nice idea!
Rabbi Rashi Simon