
He is barking because you are excluding him, and he eventually get syour attention, so he knows he has to be really persistent to finally be let in.
You will probably find it easier to address the reason for excluding him and train him to behave better around visitors rather than let this problem get worse.
When your neices call why not have him on a lead with you until he calms down (or until they leave). This way you ahve control of his behaviour and he will calm down after a while and realise what behaviour is accepotable, and get into the ahbit of being clam and well behaved, as he won't have any choice.