Maybe you should trying doing a flight with your little one before this to see what the reaction is.
As for business vs economy. It depends on how your child reacts. Business is best for sleeping not really extra room for the child to play. If your airline has bulkhead Premium Economy that may be best taking up all 3 seats. That way your child has a seat and a parent on each side and some room ahead for easy access to get out and make diaper changes and walk her around.
I've been flying since I was 4 months old and many of my childhood flights were 14 hours. I experienced ear problems and motion sickness when we circled before landing so many of the people seated near us were not my biggest fans when I was young and would cry because the ear pressure hurt. But this was when I was an infant/toddler.