I think it will be easy for the kids to pick up mandarin easily, especially nowadays kids were intorduced to mandarin at a young age. I once attended a talk by an experienced chiense teacher. He himself is native mandarin speaker, he said is is acceptable for native cantonese kids to speak mandarin with a slightly cantonese accent. Even in China, there are so many different types of mandarin accent. The same is for English.
I think the most important is to let the kid enjoy learning chinese at school and also don't hamper them from learning either mandarin or cantonese. most important for language is to encourange the kids to read lots of books.