Samsung tablet locked with power on my daughter has forgotten the password for her tablet. Not an issue if the device was off but it is on. Any suggestions?
Resetting the device will do the job. But you'll lose all your data. There's also a risk of google account authentication requirement on reboot after you format in certain devices involved so continue at your own risk.
Here's how to do it:


Press and hold the power button for 8 seconds, and the device will power off automatically.
Reboot the device by pressing and holding Volume down + Power button at the same time 
Now, you can format your device from the recovery mode. Navigate with the volume keys and choose wipe data/factory reset. Press the power button to confirm.
Your device will reboot after formatting just like a new device and logging in back to your Google account will restore most of the data if you had backed up before.