I know the feeling about travel guilt; I travel quite a bit with my job and when I have to stay over a weekend somewhere far away I feel guilt for planning my whole time riding!
But like others have said, sure you’ll miss each other and I agree with the statement that it builds the relationship bond. Certainly find that with my family anyway.
The only thing I miss when doing this type of thing is actually sharing the experience with someone, your own memories are great but when you share the experience it’s defintately a greater feeling and the years after. Take them with you and let them explore NZ as well, from the comfort of a camper van or something, your backup crew 😆 And your 15YO will probably come back with a different view on life 😉