Customs Question

You will go through immigration at the point of entry into the Schengen area i.e. Paris.

Is this one ticket: US via Paris > Madrid? If so, your luggage is checked through to Madrid and if you have any goods to declare that will be done at customs in Madrid.

If this is two separate tickets customs will be at Paris. I'd leave at least 4 hours in this case

1. Yes and no. You'll go through passport control and security. Customs (which deals with importation of goods, not people) will be at Madrid. You should be precleared on the COVID requirements before departing the US. If you don't meet the requirements, you won't be allowed on the plane. Additional checks will depend on the requirements at the time of your flight.
2. Probably. Use the CDG website to find out where your flights arrive and depart. https://easycdg.com/passenger-information/transit-connecting-flight-connections-paris-cdg-airport/
3. Hard to say. Will your flight be in time or delayed? Will your entire trip be on 1 ticket or 2? If the latter, you may have to leave airside, get your luggage and re check them and get checked in for your second flight, then clear security again. Which could add at least another hour. I'd probably want 3-4 hours if gambling with 2 tickets.

The easiest solution is to book your whole trip on one ticket. If you miss your connecting flight, the airline has to get you to your destination on the next available flight. If its a separate ticket, and you miss your flight, you're SOL.

Will we need to transfer terminals from our international flight to our inter-European flight (France to Madrid)?

Hard to say, since you do not know what airline you will be flying, but generally, yes, you will need to head from International arrivals to at least the Domestic side of a terminal, if not an entirely different terminal.

How much time would you suggest we allow between our arrival into CDG and our departure to Madrid?
Thank you very much!

Since you know your arrival time, but not other flight information, you must already have your ticket, so looking at buying a second ticket. The response is, how lucky do you feel? Your second flight is entirely disconnected, so if you arrive late, or do not leave enough time, then you are stuck without a flight. I tend not to do this, but taking 2-3 hours to get through immigration and over to your new gate is not unheard of at CDG, it is usually faster, but you never know. If you have bags to retrieve and recheck, that means leaving the secure zone, and going back through security, another big unknown. Will you need to check in for your new flight?

You might look at your carrier from the US to CDG, find a flight with them, and call and ask about linking the itinerary, that way you are guaranteed to get to Madrid if something happens. Otherwise, leave 3-4 hours, and consider buying refundable tickets. I also suggest not checking bags, carry-on only, and work with a carrier that has online check-in, even if you need to download their app and set up an account.

Thank you for all the thorough responses! We so appreciate all the information. Since we have already purchased our tickets from the US to Paris it sounds like we need to leave ourselves plenty of time! We are a family traveling and we are planning to only have carryon luggage (backpacks) so we can move swiftly. Lots of great questions to consider and love the idea of refundable tickets and doing online checkin. Thanks again!