I fully agree with Boring Panda's answer and upvoted it. Statistics probably won't help. This answer is not meant as an alternative to the accepted one but as something to consider in addition to it.
Is there a statistic that makes kids feel better?
I don't think so. But here's one that might at least put the risk into perspective for your kids: About 0.2 out of 100'000 people die because they're struck by lightning. About 20 out of 100'000 people die in traffic accidents in Brazil. About 200 out of 100'000 people currently seem to die with COVID-19 in Brazil. That's 10 times the traffic accident rate, and your wife isn't young and therefore carries a somewhat larger risk than others, but consider:
Have you ever worried that you'll die because you're struck by lightning? Probably not.
Well, then: Do you ever worry that you'll die in a traffic accident because that's 100 times more likely than being hit by lightnig? Probably not. But why not?
And I bet you don't really worry all that much about dying in a traffic accident, anyway, even though you cross the street and possibly use public or private transportation daily.
So shouldn't the same thing hold when you worry about dying with COVID-19? You can put this another way: For every 10 days worrying that their Mom will die with COVID-19, your kids should put in a day where they worry about their Mom dying in a traffic accident. If they don't, they're overestimating the risk presented by COVID-19, or underestimating the risk presented by traffic accidents.
Besides worrying about COVID-19, your kids could also worry that their mother will die of a heart disease, which (at least in the US) also kills about 200 out of 100'000 people. And the risk of dying of cancer is actually several times higher. So why don't they worry about these things?*
I don't mean to say they shouldn't have a right to be worried. Boring Panda is absolutely right there. But people aren't very good at estimating risks - you could show your children that we regularly overestimate most risks we hear about often and completely ignore others that are actually comparable or even higher, but not as present in the public discussion. This will give them a way to calm themselves with logic if they're so inclined.
And we ultimately do all die because of something - we can hope it's old age, but I can get run over by a car tomorrow, or die by a stroke next week. So what the statistics actually mean (at least to me) is that we should use the days we have to do what is important to us, because there's no guarantees life will last. That might also be something worth teaching your kids: Seize the day!
*Of course, the heart disease/cancer rates are computed over a whole human lifetime and differ by age, while COVID is happening now, so that might be a legitimate reason to worry more... but you probably shouldn't dwell on that when trying to calm down kids.