Help with drafting a letter

[Theopenwindows]

I am drafting a letter to be signed by my h which will give me permission to travel with my children (currently we live in his country). I want the contents of it to last forever so I don't have to get him to sign another letter every time we go to see granny (if he's not coming with us) and I'm not sure how to phrase it. Would it be something 'The contents of this letter, including but not limited to the permissions contained in it, are intended to last in perpetuity'

???

What is "his country"?

It's an eu one, and a schengan one. Sorry, I don't want to give too many personal details away!

Don't know how anyone can help you if you don't spill the beans. You see, it's determined by the law of the country in which you live.

Oh, I see. Perhaps I could send you a PM instead? I'm really unwilling to reveal too many personal details on here (bad experience).