My children inherited part of my rental property. Do I include their rental income in my return, if so how? Or do I need to file a separate return for each of them?

If your children are earning enough income to have taxes owing, then they would have to report it on their own tax return. This applies even if they are still children. This FAQ has more info: https://turbotax.community.intuit.ca/community/tax-filing/help/are-minors-those-under-18-required-to-file-a-tax-return/00/873418

Even if they don't need to file a tax return, then you would still not include their rental income together with yours.