At this stage the impact from the US Government "Partial" Shutdown is minimal to zero for air travelers.
Staff such as TSA, CBP (including pre-clearance facilities such as in Toronto) and Air Traffic Control are all considered "essential employees" and thus are still working regardless of the partial shutdown, although they are not currently being paid (although they WILL be paid back-pay once the shutdown ends).
As you've pointed out, some TSA workers are "calling in sick", or in some cases even looking for new jobs, which is potentially leaving them a little short-staffed, however as this stage the impact of that has been at most minimal. eg, Miami airport did close the "Terminal G" area early over this weekend, however that just resulted in flights that were due to use that terminal using a different one instead with no real traveler impact.
Air Traffic Control is still operating normally, and there haven't been any reports I've seen of this changing in the (near) future.
As time goes on the impact may increase, and it certainly wouldn't hurt for most travelers to arrive at the airport a little earlier than normal just in case.
Specifically for your parents, they will pass through Immigration/customs at Toronto airport, and will NOT need to re-clear security in Chicago so even if there are delays they will not be affected during transit.
Update: As the shutdown continues, there has been some impact at a small number of airports due to a shortage of TSA staff reporting to work. TSA is still reporting that the vast majority of passengers (93%) took less than 15 minutes to get through screening, and almost all (99.9%) are taking less than 30 mins, however there have been reports of a small number of airports having longer waits. If flying, it would be best to check the news beforehand to get an idea if your airport is affected, and allow extra time to get through security just in case.
There have been significant flight delays/cancellations in certain locations over the past week or so, however these have been due to extremely cold weather and storms, and are not related to the government shutdown.