iCloud Backup of Mistakenly Locked iPad

No. It’s too late to backup now if the iPad is locked. You will have to restore it, which erases the device, in order to be able to use it.

See this for help.

If you forgot your iPad passcode - Apple Support

whose passcode was accidentally changed by my toddler child?

If other people have access to your iPad or other Apple device, change the device passcode and never share it with others.

Change your Passcode -> Use a passcode with your iPhone, iPad, or iPod touch - Apple Support

Change your Apple ID password -> Change your Apple ID password - Apple Support

• Create a strong password for your Apple ID, It is best if your password consists of eight or more characters and include upper and lowercase letters, and at least one number. You can also add extra characters and punctuation marks to make your password even stronger. 
• Never provide your password, passcode, security questions, verification codes, recovery key or any other account security details to anyone else. Apple will never ask you for this information.

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support