With [name_m]Jack[/name_m] and [name_f]Scarlett[/name_f], I like [name_m]Holden[/name_m] and [name_f]Penelope[/name_f] from your lists. Some other ideas:
[name_m]Holden[/name_m] and [name_f]Penelope[/name_f] or [name_f]Aria[/name_f] go really well with [name_m]Jack[/name_m] and [name_f]Scarlett[/name_f]! More suggestions…
I really like [name_m]Jack[/name_m], [name_f]Scarlett[/name_f], and [name_u]River[/name_u]. They’re all strong names with different feels to them but they flow well. I also like [name_m]Jack[/name_m], [name_f]Scarlett[/name_f], and [name_u]Harper[/name_u] best.
[name_m]Cole[/name_m], one syllable to go with [name_m]Jack[/name_m]. Or [name_u]Harper[/name_u], 2 syllables and trendy/cool like [name_f]Scarlett[/name_f].