So I dug through
Pub 503 today and found some things of interest.
There is significant discussion on the limit being a yearly limit, and the section on Dollar limit page 11
Dollar Limit
There is a dollar limit on the amount of your work-related expenses you can use to figure the credit. This limit is $3,000 for one qualifying person, or $6,000 for two or more qualifying persons.
Tip: If you paid work-related expenses for the care of two or more qualifying persons, the applicable dollar limit is $6,000. This limit doesn't need to be divided equally among them. For example, if your work-related expenses for the care of one qualifying person are $3,200 and your work-related expenses for another qualifying person are $2,800, you can use the total, $6,000, when figuring the credit.
Yearly limit. The dollar limit is a yearly limit. The amount of the dollar limit remains the same no matter how long, during the year, you have a qualifying person in your household. Use the $3,000 limit if you paid work-related expenses for the care of one qualifying person at any time during the year. Use
6,000 if you paid work-related expenses for the care of more than one qualifying person at any time during the year.
Example 1. You pay $500 a month for after-school care for your son. He turned 13 on May 1 and is no longer a qualifying person. You can use the $2,000 of expenses for his care January through April to figure your credit because it isn't more than the $3,000 yearly limit.
Example 2. In July of this year, to permit your spouse to begin a new job(note, seriously? permit?), you enrolled your 3-year-old daughter in a nursery school that provides preschool childcare. You paid $300 per month for the childcare. You can use the full $1,800 you paid ($300 × 6 months) as qualified expenses because it isn't more than the $3,000 yearly limit
It seems that it's not prorated, and that as long as we are both employed, expenses for those periods are fully covered.
I also thought it was interesting to see the "tip" because ordinarily I thought for the tax credit for 2021, it was limited to $8,000/kid. Under normal circumstances per this publication, it explicitly states here that you can spend more than $3k on one kid and claim up to the maximum of $6k for 2 kids even if one was under $3k. I assume that this will apply to the $8k/$16k for 2021?