I think [name_m]Leo[/name_m], [name_u]Ruby[/name_u], and [name_f]Kate[/name_f] compliment the siblings’ names perfectly. [name_f]Ella[/name_f] and [name_m]Jack[/name_m] are both short, popular, classic, often used as nicknames for something else, and very English. In comparison, [name_m]Cohen[/name_m] is too unusual (not to mention offensive to Jewish people), [name_u]Carter[/name_u] is too surnamey, [name_u]Charlie[/name_u] is too informal (unlike [name_f]Ella[/name_f] or [name_m]Jack[/name_m], it is still primarily seen as a nickname; most people would assume it was short [name_m]Charles[/name_m] or [name_f]Charlotte[/name_f]), [name_m]Beau[/name_m] doesn’t feel like an established name, and [name_f]Vivienne[/name_f] is too frilly and [name_m]French[/name_m].
I suggest:
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_m]Liam[/name_m]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_u]Max[/name_u]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_m]Harry[/name_m]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_u]Finn[/name_u]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_u]Rory[/name_u]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_u]Theo[/name_u]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_m]Luke[/name_m]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_u]Gus[/name_u]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_u]Kit[/name_u]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_f]May[/name_f]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_f]Lily[/name_f]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_f]Amy[/name_f]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_f]Liv[/name_f]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_f]Chloe[/name_f]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_f]Lucy[/name_f]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_f]Hannah[/name_f]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_f]Sophie[/name_f]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_f]Ada[/name_f]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_f]Thea[/name_f]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_f]Rose[/name_f]
[name_f]Ella[/name_f], [name_m]Jack[/name_m], and [name_f]Zoe[/name_f]