Questions re: Non-resident returning to Canada for Education

Hi There,
My relatives left Canada and worked in Asia in the past 15-20 years.
- They did not declared as non-resident at that time, but have only returned once for a one-week visit in about 14 years ago.
- They don't have any bank account, property here.
- Their Driver's License & OHIP cards were all expired for a while and were not renewed.
- They have 3 children, one attending high school and two attending primary school. They all have Canadian passports.
- The parents did not file tax in Canada in the past years.

Questions:
1) Can the children come to Canada and attend school? If so, do they need to pay tuition? If so, in-province?
2) If the mother returns with the children, will she need to file taxes for the years she has been away?
3) If the father continues to stay aboard, what is the tax implication?

Thanks in advance for your reply!

Regards,
M.