Temporary Residents Visa dependent child applying for permanent residence

[srb]

Hi
My granddaughters (aged 12 and 4) are going to Canada on TRVs (I think the visa will be 6 months) with their mother who is a Canadian citizen and planning to move back permanently at the same time. Can they apply for permanent residency going on to Canadian citizenship whilst they are there, and therefore stay on with their mother who is not living with their father but who as written a notarised letter allowing them to "visit". Are there any legalities here? Otherwise to get a Permanent Residency from where they live will take 2 yrs whereas if they were in Canada probably around 12-18 months.

Thank you

Suzanne