no brainer Cantonese, OPOL at home Cantonese and English. Written language the same, however different versions of the same fundamental script. Once bilingualism is established verbally, next comes the writing/reading, then at that point you could add Mandarin as a 3rd spoken language, how you do that not sure. Going to a part-time school of Mandarin is NOT going to make children bilingual, but a parent speaking Cantonese as first language will.
If possible, can your dp speak Cantonese and only Cantonese with the children at all times? This will mean they will be bilingual, which is a fantastic advantage, whatever you decide about the school. The advantage of the school would be that they would learn to read Mandarin (which is, I think, more or less the same as written Cantonese) as well as speak it.
I'm sure other parents with specific experience of this will be along to give more advice. Are you in the UK? Is there a big Cantonese community near where you live? What opportunities other than the whole of school are there for your kids to speak Cantonese/Mandarin with others?
Our children are half Chinese, my partner is from Hong Kong and speaks Cantonese. We have been looking around for Chinese schools but it seems that most of them do not offer Cantonese any more, just Mandarin. Bearing in mind that Mandarin is spoken by a large number of people I wonder what is best for our children. How close are the two languages? Should we make them learn Mandarin and try to learn it alongside them?