I'd give the oldest of the kids my B1 and have him or her just stand with DH at B5. B2, B3 & B4 won't mind. That way, at least the kids will all be with one of the adults for boarding. Of course, if they are allowing family boarding between A&B, DH & youngest could just board then.
If you A boarders go towards the middle or back of the plane, it's almost a sure thing that you all will be near each other,since DH & child will be along very shortly.
FWIW, No one will care if you save a middle seats towards the middle or back of the plane.