Cannot type anything other than numbers on lock screen of remotely locked device

You wouldn't think this would be possible, but I used Android Device Manager to remotely lock my kid's tablet, and I used a password that includes upper and lower case letters and an underscore. Trouble is, I cannot unlock the device because it only allows you to type numbers on the lock screen.
Do I really have to flatten the device now?

I tried to remotely change the password, but I can't connect to it remotely anymore! I don't know why. It's powered on, but apparently doesn't want to connect to my Wi-Fi anymore.

Did you change the password on your wifi?

Was the tablet set for a PIN? That would still apply, but you changed it to a password (someone wasn't thinking when they allowed the site to do that), If so you're basically out of luck, as far as I can figure.