Money transfer app for teens?

Yellow_mango

I have two sons aged 14 and 16. They each have an appropriate teen bank account (one with Barclays, the other with Santander). However neither banking app allows them to transfer money to another account (for example to re-imburse one another or a friend for something). I think it might be possible from the desktop banking site, but they can't remember login details after setting up the app. Paypal is not available until age 18. Is there another option which would allow this to be done from a smartphone? I'm just talking small amounts between friends, like to split a meal or something. 
Thanks!