"If spouse died in 2019"

The Pandemic is a 2020 issue.

Are you asking if they need to repay an amount sent based on the prior years' tax returns which won't apply to 2020 actuals?

https://www.aarp.org/money/taxes/info-2020/stimulus-check-in-deceased-name.html

https://www.irs.gov/newsroom/economic-impact-payment-information-center-topic-a-eip-eligibility

https://www.irs.gov/newsroom/economic-impact-payment-information-center-topic-i-returning-the-econom...

"or a child was born in 2020, will that change the calculus?"

The 1040 form will include the provision for the additional amount for this child, if that was not included in the stimulus payment and your client qualifies.

And they have until Saturday Night to file and include this info Now, of course. The deadline hasn't passed.