I love Xander though not so keen on Brent. I think Frankie is nice but only as a nickname for Francesca which I'm not sure 'goes' with your other children's names. Frankie by itself is top 300 though so, though I've never met one, obviously a perfectly acceptable standalone.
I second the suggestion of Savannah but I'm biased 
If you like Casey, Nathan, Caden and Nikita, how about:
Girls:
Amber, Anya, Abbie, Alexa, Ashley, Bailey, Briley, Chloe, Cassidy, Clea, Callie, Danielle, Ellie, Evie, Elodie, Emery, Farrah, Faye, Gianna, Genesis, Georgie, Hayley, Harper, India, Jade, Jordan, Jayla, Kayla, Kaylie, Kelsey, Kyra, Kia, Kendall, Lyla, Layla, Leah, Lacey, Lexie, Lucy, Lily, Macey, Marley, Morgan, Madison, Makenna, Paige, Paisley, Robyn, Roxanne, Riley, Sadie, Sienna, Sasha, Tegan, Taylor, Tia, Zoe, Zia.
Boys:
Aaron, Austin, Ashton, Ashley, Adam, Brogan, Brennan, Brandon, Cole, Caleb, Callum, Cameron, Dylan, Declan, Damon, Damien, Dominic, Evan, Ethan, Finn, Harley, Hayden, Jake, Jordan, Jayden, Jared, Kyle, Kieran, Kai, Lucas, Logan, Milan, Owen, Parker, Preston, Rhys, Ryan, Sam, Tyler, Tian, Zach