how do I sign in under a kids account? I

If the kid is 13 or over, use How to create a new Apple ID https://support.apple.com/en-us/HT204316.

If the kid is age 12 or younger, you'll new to set up family sharing. See:

Family Sharing - Apple Support

Set up Family Sharing – Apple Support

Family Sharing and Apple ID for your child - Apple Support

Family Sharing and Apple ID for your child under 13 years https://support.apple.com/en-us/HT201084

Set up - your child’s iPhone, iPad, or iPod touch

When changing to the new ID, there may be some data loss, but you can minimize it. If syncing photos with iCloud, ensure Optimization is turned off in Settings/Your Name/iCloud/Photos and/or in Photos/Preferences/iCloud. Then make sure full resolution photos are on the device/computer. When signing out of iCloud, if asked whether you want to retain data, choose yes. Messages conversations will be lost if they are addressed to the ID. See the article below for other places that the old ID should be removed. See Winston Churchill's response in the below discussion. Also set up Family Sharing between the IDs to keep access to your apps/purchases. 

How to switch Apple IDs on your iPhone or iPad.      

How to transfer all files from one iCloud account to another.        

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