Hi Courtney,
 
I can respond to you from personal experience.  The year before kindergarten, my two daughters went to a French speaking babysitter, who spoke French to her children and English to mine.  Truly a bilingual experience!  Plus, my girls continued in French Immersion schools throughout grade twelve.
 
Yes, it would be more challenging for your daughter to learn another language.  In the city where I live, the children who were in French Immersion classrooms were, by and large, children who were more able language learners, so it was a good challenge.
 
My husband spoke French so that helped with homework, but, I must admit that I often felt frustrated because I couldn't read with my children in French.  Nor did I know how hard the books were that they were choosing from the library, etc.  Actually, my husband also had this difficulty because he wasn't certain, at each grade level, how much students should know.  So, this will likely be a frustration for you.
 
Where I live, every year the children take one or more classes in English, so I was able to keep check on my children's progress that way.
 
I decided early on that if my children were falling behind in their English development, I would take them out of French Immersion.  That wasn't something that the French teachers would necessarily have recommended, but I felt that English was the more important language to master.  In the end, I didn't remove my children from the French classrooms because I saw that they were learning both.  And this facility in two languages was what I had desired in the first place.
 
Learning the vocabulary of another language is a great challenge in the beginning.  You might want to take a "Learn French" class yourself.
 
If you don't live in a community with many French speaking people, then your daughter won't have much opportunity to practice speaking French, except in school.  The school might arrange field trip experiences to French speaking communities and countries.  
 
I'm a little concerned when you say the school follows the curriculum from France.  The school district where I live uses the same curriculum in French and English.  I think that you should compare the curriculum of this school and your state curriculum.  Perhaps get an educator to take a look with you.
 
In the end, my daughters are fluent in English and French, and have high school Bilingual certificates!!!  They also have found it easier to learn other languages, such as Spanish.  This is the advantage that Immersion educators have always spoken about.
 
The best to you and your daughter.
 
Sylvia HS,
Reading Specialist, Author