Share iTunes purchase
Apple offers Family Sharing, which is intended for this use.

Apple Support HT201088

Start a family group

On your iPhone, iPad, or iPod touch:
Go to Settings > iCloud.
Tap Set Up Family Sharing, then tap Get Started.

Confirm that you want to be the family organizer and that you're signed in with your personal Apple ID.
Follow the onscreen instructions.

Invite people to join your family

On your iPhone, iPad, or iPod touch:
Go to Settings > iCloud > Family > Add Family Member.
Enter your family member's name or email address.
Follow the onscreen instructions. 

Join a family group

You can accept or decline an invitation to join a family from your device:

On your iPhone, iPad, or iPod touch, go to Settings > iCloud > Invitations.

If the invitation is sent to your email address, you can respond right from your email.
To accept an invitation, follow the onscreen instructions. Here's what you'll be asked to do when you join:
Confirm your Apple ID and password.
Choose if you want to share your content and which Apple ID you want to share from.
Choose if you want to share your location. If you agree, your location can be displayed on family members’ devices in Find My Friends and Messages, and the location of your shared devices will also appear on family members’ devices in Find My iPhone.