I actually did this for about 5 years (5 years ago), specifically making games in Scratch. I’m wary about sharing too much personal information in a comment thread so feel free to message me if you’d like to discuss further, I’d be happy to share context that might be relevant.

Here’s a few key things I learned.

• Get them to interaction as soon as possible. It’s better to start with a template game that is “playable” but has room for modification e.g. movement speed
• Age is little indicator of aptitude, I saw 5 year olds speed past kids twice their age
• Set expectations for what can be done: in scratch; in the time you have; by a single kid. They don’t necessarily know they won’t be able to make a massive 3D open world game
• Few kids I saw were excited by programming in and of itself but plenty took to it as a means to an end. They might be way more into making a cool animation and they’ll learn how to programme an animation. You have to figure out what they want to do and then show them how to do it by programming
• Not specific to programming but kids have lives and you have to be prepared to deal with that. They will not all come to you ready to learn, they might be dealing with any number of other things, and they won’t leave you unaffected. This can be as difficult as it is rewarding but it’s ultimately why I left working with kids. I don’t want to scare you off but I saw a lot of adults completely unprepared to be in a child’s life and you very well could be. I’d even go so far as to recommend seeing if you could get someone involved who works with children professionally. I have a lot more on this specifically but down want to pour too much cold water on you.

Edit: I forgot to add, I did this for a living, not a volunteer