I was among those who thought it was unnecessary with middle names, but if you really (I wonder why?)want your child to have one then it of course your opinion which counts and not mine. I would then prefere the middle name "Ivy" since it is easy to hear which letters it consists of. Belle will probably be pronounced as Bell and then why add that extra e?
Perhaps it is just I, but I think that names are to be clear with as few odd spellings as possible. That will also make it easier for your child to learn to spell his/her own name.