English has very few diacritics to the extent that English language keyboards often do not support diacritics. If [name_f]Amelie[/name_f] were to live in the English speaking world, I wouldn’t be surprised if the diacritic were dropped frequently. If your other children have three given names, I would add a second middle name.
I prefer [name_f]Amelie[/name_f] [name_f]Constance[/name_f]. The first thing that came to mind as a second middle names was [name_f]Octavia[/name_f] but unless you’re really considering the combo as a child’s name, I think you can just leave it as it is.