I'm repeating a few things but maybe saying them in a different way will be helpful...
You and your spouse can both gift each person (daughter and son-in-law) $15,000 in 2020 without any tax consequences.
You: $15,000 to daughter + $15,000 to SIL = $30,000
Spouse: $15,000 to daughter + $15,000 to SIL = $30,000
Total: $60,000 If that's not enough you can do the same on January 1, 2021.
The mortgage company may very well look back to see you gave the money. You and your spouse will then have to provide a letter indicating the money is a gift with no strings attached. Back in the day, mortgage companies only looked back 3 - 6 months. I realize you're not trying to hide anything but giving the money sooner rather than waiting until nearer to the closing date may make things easier.
Do not give a "gift" with any expectation that it will be returned. If you expect it to be returned then it's not a gift.
If you want to make a loan, that's fine and the recommended method is to record it properly, along with the proper terms (interest rate, payments, due date, etc.). The IRS establishes guidelines for interest rates. For instance you cannot make a proper loan with a 0% interest rate or an interest rate that is dramatically lower (or higher) than rates established by the IRS. The idea of a life insurance policy that would pay off the loan is a good thought, but it may not be needed provided the loan is recorded in the county of record against the property. You will be an official lien holder and will be paid back. However, the life insurance would give you some "belts & suspenders" as the British sometimes say.