All power to you! Way to go! Love the letter and would include the word, "legal" to drive home the point.
There is a bit if a rub though - which changes nothing in your letter - but just to be aware of it: The elders may show your husband the letter and that may impel him to view your letter as a challenge. What if he discusses it with your daughters and frames it as "persecution" and gets them to drive the issue. Young people can be terribly self-absorbed and defiant under the best of conditions. Imagine that either or both of your daughters develop the notion that they should be able to decide for themselves they want to become baptized like all their same-age JW peers and begin to view you as an "opposer".
Paradoxically, sometimes the best way to get young people to do something is to forbid it until they are old enough - it inadvertently creates an incentive for them to want to do it right now - especially if their peers are doing it! Yikes!