DS2(9) is coming up to Grade 1 piano and has weekly lessons. He's keen on the idea of apps to supplement his usual practice because he loves anything computer-related. But so far he's only found things that are really frustrating - that promise lots but then require upgrades to payment levels before you get more than beginners stuff. We don't mind paying a reasonable sum for an app if it's good but we don't want to pay for things that turn out to be constantly hooking you in to pay more. Any recommendations for around Grade 1 level? His teacher is lovely and an excellent musician, but old-fashioned so this isn't the kind of thing she knows about.