Can I slice off a piece of the Sun? Bear with me. I want to explain this to my 6 and half year old.

If, all things being equal, we could reach out to the Sun with a suitable implement, a gigantic space knife, could we cut a piece off the Sun? Take a slice from it? Do we end up with a body of Hydrogen and Helium, or is it impossible to extract anything from it?

I know that the Sun is a massive ball of plasma, the sheer mass compressing the gas through gravity until fusion is achieved. What I have been asked is, effectively, what happens if you try to take a piece of that plasma? Of our sun?
The idea of removing material from the sun for either fuel or to lengthen its lifespan is called "star lifting". Various hypothetical methods have been suggested, including heating parts of the atmosphere or squeezing it using magnetic fields. My own tentative suggestion is to combine these with chaos control to manipulate turbulence into making coronal mass ejections in desired directions. 

The problem is not just that the sun is hot (5777 K at the surface), but that the stuff that is easy to scoop up with a huge magnetic field is a fairly sparse plasma - you don't get that much material despite the vast scales. Of course, you may want to compress the plasma into a smaller and more convenient ball, but now the ideal gas law tells you that it will heat up proportionally - to get a cool hydrogen ball you will need to bleed off all that heat slowly. 

Which is why I think Dyson spheres are probably the easiest way to start.