What’s the difference between Switch Child Device and Switch Level Child Device? I can’t seem to find any doc about Switch Level Child Device for SmartThings.
If you’re using the current version of Inovelli’s custom DTHs on SmartThings, you shouldn’t need to worry about the “switch child” DTH at all; they’re using one ST has built-in. If you’re upgrading from an older version or for some reason prefer it, then this doesn’t apply.
Regardless, the plain “switch” child devices are ones that get created for easy access to preconfigured notifications on the LZW30-SN and LZW31-SN (not sure which products you’re asking about; I think it was also used for individual channels on their old dual-outlet Z-Wave plug and probably others I’m not aware of). For the “scene and notification” products, this is probably what you’re looking for — and don’t need to do anything special (anymore) to make it happen.
The “switch level” DTH would be necessary to install from Inovelli if you want to use it, which as far as I can tell is only used if you enable the “Create ‘Default Level (Local)’ Child Device” and/or “Create ‘Default Level (Z-Wave)’ Child Device” options in the driver. These would provide a way to automate the default level (brightness) of the dimmer when operated from the paddle or an “On” command from ST, respectively. If you don’t want to automate that and just want to set the preference one time and be done, you don’t need to enable these options or install these DTHs because there’s already a preference on the device page to do that.
tl;dr if all you want is notifications and you’re on a reasonably current DTH, you don’t need to do anything. 
I have LZW30-SN. According to https://support.inovelli.com/portal/kb/articles/installation-setup-lzw30-sn-on-off-red-series-gen-2-smartthings#Installing_the_Child_Device_Handler I need to install Switch Child Device DTH to setup notifications. Are you saying the latest inovelli-switch-red-series-lzw30-sn.groovy eliminates the need for switch-child-device.groovy completely?
Out of curiosity I looked at the notification setup document for LZW31-SN, the dimmer also refers to Switch Child Device instead of Switch Level Child Device. Looking at the groovy code, it seems the difference is Switch Level Child Device creates a virtual generic-dimmer, while Switch Child Device creates a virtual generic-switch. But I don’t understand how the virtual dimmer adds any functionality to the notification.
That document looks outdated to me.
In any case, the (plain) “switch” child devices, regardless of which DTH is used to create them, are used for the notification LED settings. They create switch devices you can use to turn on the notification via standard SmartApps without having to resort to custom commands or other methods (but you do have to “pre-create” these in the device page; these are what the notification 1-5 settings will help you make). The “switch level” drivers, as mentioned before, aren’t used for this but rather for creating different child devices you can use to set the default physical or digital level. If I had to guess, I would say that usage is not very popular, and again, unless you specifically enable the option to create those devices, you don’t need to worry about them.
As far as whether the LZW30-SN DTH wants the ST or Inovelli child DTH, I looked at the latest LZW30-SN DTH code on GitHub. A few updates ago, they switched over to using built-in ST DTHs for the “switch” (but not “switch level”) child devices. You can verify this on lines 715-725 of the current version if you are interested. I would assume the docs just haven’t been updated to reflect this yet.
I’ll tag @Eric_Inovelli here in case he’s the one responsible for the installation instructions; if not, he should at least know who. (To summarize: the ST instructions still say to install the “child switch” DTH when they’re using the built-in ST component switch DTH now. Mysteriously, I also don’t see the “child switch level” DTH mentioned at all.)
Thank you for the pointers. That really helps. I checked it was a commit from February that changed to ST child device. I verified the type of my notification virtual switch is indeed Child Device, instead of Switch Child Device. Looks like both Switch Level Child Device and Switch Child Device are there to support legacy devices.
Indeed, they changed the DTH in a backwards-compatible manner so existing users could keep the “old” child devices on the Inovelli DTHs, while new users could skip the extra installation and use the built-in ST DTH, while all users could use the same current Inovelli LZW30-SN (or LZW31-SN, where a similar change was made) DTH.