We are visiting our brother in UK along with our 2 kids and my mother. We have planned to drop them there and travel back. They have planned to return along with my brother after couple of weeks. Our kids have got child accompanied UK Visa(Mine and my husband's passport# have been mentioned in visa).
Will they have problem in their return as they will be accompanied by my mother and brother?
No -- but make sure the kids have a nice letter from both parents no matter which border they cross. While not a legal requirement AFAIK anywhere it's a strong recommendation. The Canadian version will do just fine. Put yourself in the border guard's shoes: they want to know there's no kidnapping taking place (which happens more often within the family than you'd think -- think custody battles).