Just download Xcode, it’s free, and it’s from Apple. With that you can deploy any app (if you have the binary - or the source code, but you don’t technically need the source code) on your own devices and a hundred other people’s devices (it’s supposed to be work colleagues, but your kid’s tablet will work too in practice, who’s going to check or care?).
You can also “sideload” your app to up ten thousand devices linked to other Apple IDs via TestFlight which also a service run by Apple (for a nominal subscription fee) and intended for developers to test apps that aren’t ready for distribution yet, though that process does require a partial review by Apple (it’s mostly just an automated malware scan, not a full app review). It’s perfectly normal for an app to be in development for years without going public. Most apps I’ve written have never shipped, but I still use a few of them on my own devices.
As for getting a copy of YouTube++ from a reputable source, that doesn’t contain malware… that’s basically no different from downloading software for a Mac or PC. Be careful where you download it from yeah?
Generally though, using Xcode is safer than using AltStore since you haven’t jailbroken your device and all the sandboxing/etc is still in place. I’d be more worried about malware infecting your Mac when you load it into Xcode than I would about the iPhone (though it certainly could contain a zero day that escapes the sandbox).