how do I switch to child account on ipad

Outside of Educational and Enterprise establishments, iPad and iPhone are principally intended to be a single-user device - and are not intended to shared in the same manner as you may be accustomed when using a Windows PC or Mac computer - which has multiple user profiles/accounts.

For managed environments, where multi-user usage is possible, the Deployment Guide provides appropriate guidance.

Shared iPad overview – Apple Support

For unmanaged environments, be aware that if you sign-out of iCloud and other Apple services - and allow another user to sign-in - you’ll not be able to change back for 90-days. So as to prevent account abuse, some critical Apple services will only allow one change of signed-in AppleID (on a device) in a 90-day period.

In conclusion, unless you fully understand and accept the implications, you would be well advised to reconsider any attempt to share your iPad with another user.