How do I make a separate iCloud storage plan for my child?

The only way to accomplish that is to set the child account to an age over 13 and have them leave family sharing momentarily, then activate the new iCloud+ storage on their account directly, and then have them join family sharing again.

At that point, the storage on their account will remain active, as long as they don't choose to use the family storage.

click here ➜ Share iCloud+ with your family - Apple Support

If a family member is already paying for their own iCloud+ plan, they can choose to switch to your plan or keep their own and still remain part of the family.

Alternatively, it may be a better option to manage their storage usage and delete things that do not need to be on iCloud and as such release some storage space.

Why?

Apple Accounts under 13 years of age must be part of Family Sharing.

As are part of family sharing they can use any iCloud+ storage you are subscribed to, but their files and photos remain separate to yours. And they won't be able to see your files or photos.

You cannot give them an entirely separate iCloud account unless it's set to an age 13 or over. But not sure I see the value of doing this, and it will just be very confusing to manage.

Why do you want to give them their own storage plan?