We're sailing from Southampton (not Malta!).
Ok so it sounds like I need to buy 4x LFTs from Randox or similar - one for me (vaccinated), one for wife (vaccinated), one for mother in law (vaccinated), 1 for 8 year old and none for 4 year old. The second test required for the unvaccinated 8 year old appears to be offered free of charge at the terminal:
"In addition to the above antigen test, all unvaccinated guests aged five to 11 years will also be required to take a complimentary lateral flow/antigen test once they arrive at the terminal, administered by our service provider, Prenetics." https://www.pocruises.com/cruise-with-confidence/preparing-to-sail