Can I have two apple Id's, one for each phone?

Yes. Depending upon the age of your granddaughter, the second ID may need to be set up under Family Sharing.

This document explains how to create a new Apple ID (create an Apple Account) --> How to create a new Apple ID - Apple Support

Follow the instructions in this support article to create an Apple ID for a minor child - Create an Apple ID for your child - Apple Support

I am surprised the article doesn't mention these but also look into Screen Time as part of parental controls, and Ask To Buy.

Use Screen Time on your iPhone, iPad, or iPod touch - Apple Support

Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

Approve what kids buy with Ask to Buy - Apple Support

Set up parental controls with Family Sharing on iPhone - Apple Support

A related issue will be transferring what content you can to the child's account. Merging, splitting or transferring items between Apple accounts (IDs) is for the most part not officially supported. Read the links below for things you may need to do to transfer as much as is possible and to share some others.

Apps and media bought with one ID are permanently associated with that ID. Look into using Family Sharing to continue to be able to use items purchased with one Apple ID with other Apple IDs, and for some data sharing features.

- Family Sharing - What is Family Sharing? - Apple Support

- Set up Family Sharing - Set up Family Sharing - Apple Support

Share folders with iCloud Drive --> Share files and folders in iCloud Drive on iCloud.com - Apple Support

Read How to use Shared Albums in Photos on your iPhone, iPad, and Mac - Apple Support about how to share photos.

If you have signed into the first ID on a device then fortunately when you sign out of that ID, do not erase the device, then sign in with a different ID, it asks you in most cases if you wish to merge the data from the two IDs on your device (toggle the ones you want to on). Say yes and later delete individual items from the other ID that you do not want. Note, in How to save apps and progress on one Appl… - Apple Community a Community Specialist says, "you will not be able to transfer the meta data from one Apple ID to another. Once you switch to another Apple ID the information associated with the old Apple ID will be gone except for physical content on the device like photos and videos." Also turn off storage optimization before doing this. With Optimize Storage, smaller, space-saving photos and videos are kept on your device while all of your original, full-resolution versions are stored in iCloud. Make sure photo optimization is turned off. Manage your photo and video storage - Apple Support

Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac --> Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support

If another Apple ID is signed in you may need to enter the password for that ID to sign out of that ID first.

Click on this link to learn how to 'Sign in with your Apple ID' --> Sign in with your Apple ID - Apple Support