From your list, I really like [name_m]Francis[/name_m] and feel like it fits especially well with [name_f]Mary[/name_f] and [name_m]Jack[/name_m]. It has the same classic, understated feel.
I also really like [name_m]Augustine[/name_m], although I might personally prefer [name_m]August[/name_m]. [name_m]Augustine[/name_m] feels quite grand and elaborate next to the sweet simplicity of [name_f]Mary[/name_f] and [name_m]Jack[/name_m], whereas [name_m]August[/name_m] gives you that same lovely feel while still allowing for both [name_m]Gus[/name_m] and [name_f]Auggie[/name_f]. That said, if you love the full [name_m]Augustine[/name_m], I think it definitely works!
I like [name_m]Everett[/name_m], too, and especially love [name_m]Rhett[/name_m], but I agree with others that it feels a little more modern/contemporary than [name_f]Mary[/name_f] and [name_m]Jack[/name_m]. If that difference in style doesn’t bother you, though, I think it’s a really handsome choice. [name_m]Clark[/name_m] and [name_m]Francis[/name_m] are probably my favourites from your list. I do think [name_m]Frank[/name_m] and [name_m]Clark[/name_m] having the same ending sound as [name_m]Jack[/name_m] makes them slightly less appealing as a sibset, but that’s probably just personal preference and may not bother you at all.
For girls, I actually think all three work! My only hesitation with [name_f]Clara[/name_f] is that it’s quite close to [name_f]Claire[/name_f], Mary’s middle name. It wouldn’t necessarily be a problem in everyday life, but I personally might want a little more distinction between the names.
I’m also not usually a fan of repeating initials, so [name_f]Jane[/name_f] would be a little lower for me simply because of [name_m]Jack[/name_m]. And I haven’t seen anyone mention this yet, but [name_f]Mary[/name_f] + [name_f]Jane[/name_f] together does make me think of [name_f]Mary[/name_f] [name_f]Jane[/name_f] as a combination (the shoe style, [name_f]Mary[/name_f] [name_f]Jane[/name_f] [name_m]Watson[/name_m], and unfortunately the slang association), so that would probably push me toward [name_f]Alice[/name_f] if those were my three choices.
I think [name_f]Alice[/name_f] is probably my favourite of your current girl options. It has that same sweet, timeless simplicity as [name_f]Mary[/name_f] and [name_m]Jack[/name_m] without feeling too matchy.
Some other ideas I’d consider:
- [name_f]Anne[/name_f]
- [name_f]Rose[/name_f]
- [name_f]Lucy[/name_f]
- [name_f]Cecilia[/name_f]
- [name_f]Helen[/name_f]
- [name_f]Edith[/name_f]
- [name_f]Beatrice[/name_f]
. - [name_m]Benedict[/name_m]
- [name_m]Edmund[/name_m]
- [name_m]Simon[/name_m]
- [name_m]Thomas[/name_m]
- [name_m]Arthur[/name_m]
- [name_m]Dominic[/name_m]
- [name_m]Theodore[/name_m]
- [name_m]Leo[/name_m]
- [name_m]Hugo[/name_m]
- [name_m]Peter[/name_m]
- [name_m]Louis[/name_m]