Partner and son visit

Hi all, I have recently moved from the UK to Bahrain to live and work. I am planning on getting my partner and son out to visit at Christmas and given they have different surnames as we are unmarried am looking to find out if that will be an issue in terms of entry to the country and visas. I cannot access a definitive answer online and although my company will handle the process I want to understand any potential issues.

my partner is female and they are both British citizens.

any help would be greatly appreciated.

Should not be an issue if they are getting a visa on arrival.

Thanks so much . clear and concise  thanks again