I am a fail as can’t see that pattern, and I anyway don’t like repeat starting letters for first name especially when the kids are close in birth order. I agree with PP that initials AB makes the most sense in terms of balance across letters:
In addition to their suggestions, maybe:
Auden Beatrice
Arden Blythe
Adley Bernadette
Aspen Briar
Amara Beth
Airlie Bronwyn
Annika Blair