I agree with @gardenstategal that you can say “This is my first choice.” There is no need to mention financial aid (it may turn off the school), and there is no need to commit to definitely attending (since you can’t commit to that yet).
I think saying it’s the first choice would help your DD. Schools want students who are excited about the school.
Kudos to you and your DD to picking the school that is the best fit for her, even though you say it is not the ‘most popular’ school.