Excellent answers from my colleagues.  I didn't believe it the first time I saw it (many years ago) but it's explained in the 2019 2441 instructions (top of page 4):

https://www.irs.gov/pub/irs-pdf/i2441.pdf

" However, it is possible a
qualifying child could have no expenses and a second
child could have expenses exceeding $3,000. You should
list -0- for the one child and the actual amount for the
second child. The $6,000 limit would still be used to
compute your credit unless you have already excluded or
deducted, in Part III, certain dependent care benefits paid
to you (or on your behalf) by your employer."