renting home while a non-resident of canada

my son and his wife are currently renting an apartment. we are thinking of letting them move into our house when we move to the US. If they pay us rent, will this qualify as severing residential ties?

Yes. But, additionally, you must adhere to the rules for non-residential renters, like making sure an 'agent' colects the rent and remits 25% of your rent money directly to CRA.

See the CRA emigrants guide for this and all other aspects of leaving Canada, and subsequent taxation issues.