How do i lock a SINGLE aplication, NOT pictures or video, but an application like NUMBERS?

I'm not hiding pictures nor ****. I want to secure my personal and work spreadsheets from a casual user of my ipad at home. It is a work tool for me and an entertainment device for family.

Please don't suggest one of the zillions of free apps that hide pictures, video, or browser information. I'm new to the apple family and just want to simply lock my Numbers app.

I've tried the guided tour lock and it must be disabled to leave the app you're locking. Weird. You can lock access to spreadsheets only if you're in spreadsheets. When you try to leave the app and hope it'll remain locked... no dice. You have to leave the guided tour setting in order to leave the app.

I AM NEW TO THIS SHTUFF, so please give me a break. No one to ask since I am (unbelievably) the only techy guy using apple around here.