Yes. Whatsapp requires a phone number and Discord does not. The tweens who do not have a phone yet can join Discord with their siblings / friends.
The other part of this is that Discord has official University hubs, so the college kids are all in there. You need an email address from that Univeristy to join: https://support.discord.com/hc/en-us/articles/4406046651927-...
Having been out of university since before Discord was much of a thing, that's news to me. It also is eerily reminiscent of Facebook's beginning sign up requirements.
Are you using the word tweens in some sense other than its usual definition of pre-teen? My understanding is that discord, like most online services, requires registered users to be 13 years old.