I ONLY want to remove Apple mail

You don't. Mail is a part of the OS, just don't use it, or if it is not working please explain.

Load Linux or BSD. That's how you get full configuration access, and the ability and the control to maintain or delete or add... anything.

Or you can delete Mail.app, and go fully unsupported — not something that's known to work, and you'll be learning what might happen, if anything. The Mail.app stuff will get returned by various repair and upgrade procedures of course, so you'll have to delete it after recovery and likely also after upgrades. (This assuming that something doesn't expect Mail.app to be present and tip over.)

Otherwise — and this is the preferred and the supported approach — control-click on the icon in the Dock and select Options > Remove from Dock, and continue onward with whatever tools you do use, and whatever apps are shown in your dock.

Why do you want to remove Mail?

What do you hope to gain by doing that?

Then just remove the email account, leave Mail onboard but without an account it will not function.

Make her her own user account (System Preferences -> Users & Groups -> + button) and have her use that. She won't see your email or any of your other files in your home folder.

Enable parental controls on the account, and keep the admin account to yourself.

FWIW, your granddaughter will quickly know more about computing than you or I do, and will learn much about you, too. Kids are smart like that, and very perceptive. But you know this.