Unlike imagery, the technology of sound & microphones doesn't change so quickly which makes things easier.
Sound recording kit for mobile devices
Ideal setup for education settings
You need to be able to fix the microphone on a hand held pistol grip
It is also great to have a little tripod, then if everyone is busy you put the microphone on your mini-tripod and no-one has to hold it
microphone itself Must be Røde VideoMic Go II not version 1 which is £30 cheaper - around £95
cable to connect mic to device For iPads & iPhones you have to buy this Røde SC19 USB-C to Lightning Accessory Cable - around £30. If you are on Android you're in luck because a standard 2m USB-C to USB-C is fine - around £12
mini tripod Although you could just about get away with only the above, you really need method/s to support the microphone. A small tripod is a great way to support the microphone on a desk or chair. Manfrotto are pretty good - £22 (You could use thi instead of a proper pistol-grip)
pistol grip You need something for the children to hold the mic with, ideally a Røde PG1 pistol grip Cold shoe- about £11
joiner You also need this tiny little joiner, strangely sold in pack of five - about £11
wind sock One more thing to consider, is a windsock, ideal for exterior sound recording. And it makes the young people feel more professional - about £26
boom A boom is not really necessary as you have such a short cable on mobile devices.
A more professional set up
If you are in a position to spend over £300 sound equipment, you will have kit which will be more durable and produce better quality results (so reducing frustration and disappointment). In this case, go to a specialist audio supplier — with your camera — to discuss what's best for you. You can also try the internet if you're sure you know what you want.
For small bits like cables, connectors & headphones, buy two, so that, if one fails, you can carry on. Don’t let your whole shoot depend on one tiny connector
Video cameras and DSLRs come with a microphone moulded into the body and are problematic by themselves; always use an external mic with this type of camera
Some cameras still have mics you can unplug and fit an extention cable inbetween. This is a good low-tech solution and works well if you find a suitable method to cradle the microphone.
Where to go
Recording sound for filming is a specialist area so only a few places will really be able to help you. If you have a specialist HiFi shop they might be able advise and help you get what you need.
Two of the best companies to try in Scotland are The Warehouse & Canford Audio, both have many years of experience supplying sound equipment for the film and video industry. They are approachable, will understand your requirements and help you to find the best gear for your budget: they will be able to help you best if your budget is over £200
If you are out buying kit, take the camera with you.