A lesson or activity on the topic 'HTTP vs HTTPS" for 10 years old is not a lesson about cryptography. It's about privacy and staying "safe" on the web.
The EFF has a good educational material about HTTPS and Tor that you can use as inspiration (Tor is out of scope, but the part about HTTPS is relevant). The point is to explain the threats and how using HTTPS mitigates them.
As a demo, you can set up a wifi network and a laptop with Firesheep or another tool that visually displays HTTP interceptions: You can ask the kids to use their smartphones to browse websites in HTTP and compare with websites in HTTPS.
If you want to go into the details, you can tell that TLS guaranties the confidentiality, authenticity and integrity of the data coming from and to the websites. Explain those 3 words.
If you start talking about encryption keys or maths, you are going to far. But you may try to convince the teachers to do a second talk about the history of cryptography and cryptanalysis (it's usually a good way to introduce the topic).