Short foundational lessons for 7-8 year olds
If you are merely looking for feedback on this, let me point out, first, that according to Piaget's work, your students seem a bit young for this. They have only just entered the third stage of Piaget's Model. Moreover, since not all people advance in sync, some may still be in the second stage. In particular they are still very concrete thinkers. Anything that depends on abstract thinking won't be very comprehensible to them for a few more years. 

For a small exercise, however, the above might not matter too much as you aren't expecting them to do much but think "WOW". 

But there has actually been a lot of thought given to "computational thinking", through Jeannette Wing's work. This has mostly been directed at slightly older students, however, say secondary school. 

There is a well developed effort to bring computational thinking down to this age level. CS Unplugged, provides a fairly comprehensive set of activities that are targeted at 5-10 year olds. 

While there are a lot of things you can do, don't expect the kids to retain a lot of it, and certainly don't expect that they can then (or soon) abstract from it to actual computing. I find that Spiral education, in which ideas are introduced in a simple way early and then returned to (often), works well. So, for a first cycle, you give them a hint of things that might be and later reinforce that more formally. 

I find that something as simple as the song There was an old lady that swallowed a fly is interesting and fun. It is also an introduction to recursion, but at that age, don't talk about base cases, etc. 



A few more words about logo. While it is a fine language, you need to use it carefully for youngsters, so that what you do matches their cognitive level. Since they can't do much abstraction yet, there are a couple of possibilities. Since it is visual it matches their "concrete view" of the world, of course. One is to use only the language primitives. But another is to build a virtual world in logo with a set of its own "primitives" that they just use instead. These can be of a higher level than the built in commands, but are opaque to the students. Karel J Robot is like that actually. Robots obey five simple commands (move, turnLeft, etc) and students begin with only those. The robot language hides the underlying Java primitives completely.