Which education fields in French can be used in the USA?

Only the public universities are tuition free in France (you still have to pay for board and lodging plus transport of course).  Your daughters will only be accepted to French universities if they have both a very high level of fluency in the language and the IB or another Bac equivalent.  French universities are very academic.  They are also rather underfunded meaning that sometimes required classes are full so it can take a while to graduate.  Your children may opt for an academic rather than vocational subject and specialise afterwards. 

Although subjects like Law, Medicine and Veterinary are taught at undergraduate level everywhere in Europe, these are 5 year courses and the awards do not easily transfer.  Computer Science is transferable.  Some vocational degrees are rather pointless; with Business (for example) better firms prefer to do their own training followed later by an MBA.

That is at least 8 years away. 

Won’t they pick what field they want to study, what job they want? 

Any 4 year degree is accepted in the US. In Law and Medicine, everyone has to pass the appropriate test- the bar exam in law, for example. They do not have to complete an additional degree. They could, for example, apply for jobs as French teachers, and might need to complete a few graduate education courses. They could get an undergraduate degree in France, and then apply to graduate schools in the US, where assistantships often pay for the education. Most students in US universities now have financial aid, but that may have changed by the time they are ready for university. We should be investing more in education, and we may be- if people vote for the right representatives.