nicole1972 on Family Tree Circles

Hi, I was hoping to find some information before my Grandad passed away however sadly he died 3 weeks ago. I was more concerned with finding information about his Grandad who died many years before he was born. I am glad to say I found this information & discussed it with him before he died. It is now my mission to try & find anything I can about his Uncle Thomas, Aunt Betty & cousin Thomas.
Over the last year or so, he has been sharing some of our family history with me. He remembered going with some family members to the train station (mainly because the steam train frightened him) when he was very small. He was born in 1921 & I think I have found them on a passenger list heading to Wellington, New Zealand.
The names are as follows:-
Thomas Guest b 1891 departed from Southampton 1922
Betty (B) Guest b 1893 (nee Humphreys) departed from Southampton in 1923
Thomas Guest b 1916 departed from Southampton 1923
He did confirm the name Betty Guest & I have found her marriage to Thomas in 1914 & then the birth of Thomas so am almost certain these are the ones I am wanting. He was also most certain about them going to New Zealand.
The question is how do I go about finding any info about them once they arrived? In particular I would like to find any descendants if there are any. I have googled backwards and forwards but have hit a brick wall.
Is there anyone out there who can help me out?
Many thanks in advance
Gwen Humphreys.