If you can obtain a microscope with a projector/display they can all see at once, and have a bunch of familiar sample objects that get non-intuitive under the microscope, they might really like that. Especially if you can do it with progressive wide range of magnification.
For each sample, you could show it to them by eye, ask them what it is, maybe pass it to one of them, ask them what it is (including everyone shouting out the answer, like it's a game, depending on age). Then you show them on lowest magnification. Then you increase magnification. This might be new to them and break their brains in a good way.
This can get into vague overview about how big things are made up of smaller things, that look different when you look closely.
You can also talk about seeing more detail of things when you're close to them than when you're far away, which is more intuitive, though I don't know whether this will confuse them about distance and too much about optics at once.
Once they are starting to get magnification, you can also put a sample unknown to them under max magnification, so they can only see the highly magnified display of it, and make a guessing game about what it is. Progressively lower magnification, whether or not someone guesses right, so they see that progression regardless, and it also makes a reveal of the answer to the game.
For each sample, you could show it to them by eye, ask them what it is, maybe pass it to one of them, ask them what it is (including everyone shouting out the answer, like it's a game, depending on age). Then you show them on lowest magnification. Then you increase magnification. This might be new to them and break their brains in a good way.
This can get into vague overview about how big things are made up of smaller things, that look different when you look closely.
You can also talk about seeing more detail of things when you're close to them than when you're far away, which is more intuitive, though I don't know whether this will confuse them about distance and too much about optics at once.
Once they are starting to get magnification, you can also put a sample unknown to them under max magnification, so they can only see the highly magnified display of it, and make a guessing game about what it is. Progressively lower magnification, whether or not someone guesses right, so they see that progression regardless, and it also makes a reveal of the answer to the game.