Hi there!
Casting music is made easy by design and won't require your Roku PIN and this is intended behavior. Spotify has its own parental controls and there is also a Spotify for Kids option.
But, we can understand the frustration and if you don't want anything to be cast from Spotify, there are a few ways you can achieve this, but it would require changing some settings.
If you're using an Apple device, go into your AirPlay settings and change your settings here. You can disable AirPlay completely, or forget all devices and require an on-screen pin, but if the kids have access to the casting device, they can use the pin, so you'll need to disable AirPlay completely. You can find AirPlay settings on your Roku device under Home > Settings > AirPlay and HomeKit
You can also disable Screen Mirroring if you prefer, under Home > Settings > System > Screen Mirroring.
If either of those two options doesn't achieve the goal you're trying to achieve, you can create a separate wireless ID (SSID) on your router, and move your Roku TV to segment network traffic, but this means you won't be able to cast anything to your Roku TV. You can also segment network traffic without a new SSID, but that support would be beyond this forum and require a call to your ISP.
So, there are several ways to do this depending on the host device and if you want to disable casting all together or just Spotify. It could also be helpful to chat with the kids about expectations or keeping an eye on the device they're using to cast Spotify from. Hope the information above is helpful!
If anything else comes up, the Community will be here.
Happy Streaming!
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