DO WE APPLY FOR 24h TRANSIT

This itinerary is definitely eligible for 24 hour visa free transit for both of you and you don't need any kind of visa.

Of course, if you have some other stop within mainland China, we should reconsider your eligibility.

You should not worry about your 20 hour layover in Chengdu. You are advised to book your accommodation in advance and to have a printout of your confirmation of accommodation booking. This and the flight tickets printouts will be needed. You will, of course, apply for a temporary entry permit at 24 hour visa free transit counter upon your landing in Chengdu to be able to exit the airport and to have a decent sleep in your accommodation. Anyway, you should be aware that the issuing of a temporary entry permit depends upon discretion of immigration authorities on a day of your arrival.

In reality, you will not encounter problems because you will be eligible for 24 hour visa free transit and with a temporary entry permit, you will be also allowed to shortly visit Chengdu.

One more detail... In case if you don't travel on a single (through) ticket, you will have to apply for a temporary entry permit even during your first arrival in Chengdu to be allowed to collect your luggage and to perform a recheck for your flight to Seoul.