Cara -- I agree with your comment.
Many expats who see Cantonese / Chinese as such an enormous hurdle seem unaware of what most local families go through all the time.
Turn the tables. Imagine trying to give children English-language education / exposure when both parents are native Chinese speakers. They put ALOT of effort into this, from classes to tutors to trying to learn / speak English themselves. OK, it's often over the top. But if Chinese parents can make an effort to give their kids English exposure, then the opposite can be true, too. Watching my local cousins learn English -- and some of them are near-fluent in this second-language -- I saw that they had to be more independent, too. They couldn't just run home to mom and dad for homework help!
As for Canto vs. Mando -- when the kids are this young, I think it doesn't really matter. I think it's crazy when people start making decisions for 3 or 4 year olds based on which language is tried to the greater economic power (that would be Mandarin, hands down).
At this age, kids are sponges. It only makes sense that a child should have some exposure to the language spoken by 90% of the local population (that would be Cantonese). Could you imagine being an immigrant to Britain, or France, or Spain, and not having your kid learn English, French or Spanish?
With either Canto or Mando, the basics of the Chinese language will be there -- the basic structures, the characters, etc. If your kid is getting regular Cantonese exposure at a young age -- and mixing with kids with Hong Kong locals -- learning Mandarin and understanding Chinese culture will be much easier later on.