[name_m]Christopher[/name_m] [name_u]Mallory[/name_u] is great! [name_m]Charming[/name_m] and balanced - and [name_u]Kit[/name_u] is a fave of mine!! [name_m]Frederick[/name_m] nn [name_u]Freddie[/name_u] is charming too 
They both feel very classic, but with a fun nickname, so maybe some siblings could be:
[name_f]Susannah[/name_f] nn [name_u]Sunny[/name_u], [name_f]Annie[/name_f], [name_f]Sanna[/name_f]
[name_f]Matilda[/name_f] nn [name_u]Mattie[/name_u], [name_f]Millie[/name_f], [name_f]Mila[/name_f], [name_f]Mimi[/name_f], [name_f]Mitzi[/name_f]
[name_f]Elizabeth[/name_f] nn Ezzie, [name_f]Eliza[/name_f], [name_f]Effie[/name_f], [name_f]Betsy[/name_f], [name_f]Libby[/name_f]
[name_f]Josephine[/name_f] nn [name_u]Josie[/name_u], [name_f]Joanie[/name_f], [name_f]Effie[/name_f], Seffie, [name_u]Jojo[/name_u]
[name_f]Eleanor[/name_f] nn [name_f]Nell[/name_f], [name_f]Nella[/name_f], [name_f]Nora[/name_f], [name_f]Leni[/name_f]
[name_f]Harriet[/name_f] nn [name_f]Hattie[/name_f], [name_f]Hettie[/name_f], [name_m]Hart[/name_m], [name_f]Etta[/name_f]
[name_f]Alexandra[/name_f] nn [name_u]Lexi[/name_u], [name_u]Allie[/name_u], [name_u]Landry[/name_u], [name_f]Lexa[/name_f]
[name_f]Georgiana[/name_f] nn [name_f]Gigi[/name_f], [name_f]Ginny[/name_f], [name_f]Gia[/name_f]
[name_u]Benjamin[/name_u] nn [name_m]Benji[/name_m], [name_u]Jem[/name_u], [name_m]Benny[/name_m]
[name_m]Nathaniel[/name_m] nn [name_m]Nate[/name_m], [name_u]Tate[/name_u]
[name_m]Leonard[/name_m] nn [name_u]Leo[/name_u], [name_u]Lennie[/name_u], [name_m]Ned[/name_m]
[name_u]Julian[/name_u] nn [name_u]Jules[/name_u], [name_u]Jay[/name_u]
Some middles for [name_m]Frederick[/name_m] (some are a similar in vibe to [name_u]Mallory[/name_u], some inspired by the honour names, some just general):
[name_m]Frederick[/name_m] [name_u]Delaney[/name_u] (inspired by Elaine)
[name_m]Frederick[/name_m] [name_u]Cassidy[/name_u]
[name_m]Frederick[/name_m] [name_u]Hartley[/name_u]
[name_m]Frederick[/name_m] [name_u]Wren[/name_u] (inspired by Robbin)
[name_m]Frederick[/name_m] [name_u]Marlo[/name_u]
[name_m]Frederick[/name_m] [name_u]Jude[/name_u]
[name_m]Frederick[/name_m] [name_m]Sylvan[/name_m] (inspired by Sy)
[name_m]Frederick[/name_m] [name_u]Clayton[/name_u]
[name_m]Frederick[/name_m] [name_u]Ellis[/name_u]
[name_m]Frederick[/name_m] [name_m]Hatton[/name_m] (inspired by Hattie)
[name_m]Frederick[/name_m] [name_u]Peyton[/name_u]
[name_m]Frederick[/name_m] [name_u]Adair[/name_u]
[name_m]Frederick[/name_m] [name_u]Griffin[/name_u]
[name_m]Frederick[/name_m] [name_u]Rue[/name_u]
[name_m]Frederick[/name_m] [name_u]Merritt[/name_u]
[name_m]Frederick[/name_m] [name_u]Murphy[/name_u]
[name_m]Frederick[/name_m] [name_u]Mackay[/name_u]
[name_m]Frederick[/name_m] [name_m]Macauley[/name_m]
[name_m]Frederick[/name_m] [name_u]Orion[/name_u]
[name_m]Frederick[/name_m] [name_u]Blythe[/name_u] (inspired by [name_m]Gilbert[/name_m], via the [name_m]Gilbert[/name_m] [name_u]Blythe[/name_u] association)