how to turn a childs phone off using family share

There is no way to prevent the child from making calls other than to take the phone from him or her. Parental Controls in Screen time will not work for that. In an emergency situation you would still want the child to be able to make a call.

None of this is managed with Family Sharing. That is a completely different service. You are thinking of Screentime.

Read the link I posted earlier for information about things you can restrict. If you have problems with the child using the phone at night, make them give you the phone before the child goes to bed.

That is not possible.

Use parental controls on your child‘s iPhone, iPad, and iPod touch - https://support.apple.com/HT201304

You can do some limited things using parental controls but I do not think it includes turning off an iPhone remotely.

Bucket43 wrote:

Ok oh can you not allow child to make phone calls during down time using family share. I have restricted all the apps but my child is still able to make phone calls

You might want to check to see what services your carrier offers for managing children's phones. Many have add-on services that allow you to have more control over things like calls.

There is no way to turn off an iPhone remotely. You can erase it using Find My iPhone, but you cannot power it down without actually doing it on the phone itself.