To add to what u/undetermined_outcom3 said, you can still print them but let them know the timeframe for getting them will take longer. You don’t have to just come out and say “hey I can’t afford that now” but can say something along the lines of “due to business constraints it may take a month or two to get the prints delivered, but let me know what you want printed and I will keep you updated when they come in and get shipped for delivery.” That way it’s still clear that you can’t to it at the moment, but also still will honor the contract even a few years later.
And depending on what they want printed and at what size, it may not cost all that much anyway. So if you say that it may take a few months, and can then deliver within a month, that just makes it look like you’re making even more of an effort than required by the contract.
I don’t do much family photography outside of my own family, but in general if you’re giving prompts it’s not going to look candid.
If you want candid, be a fly on the wall while they’re doing their own thing. It’s probably not going to work for any short or timed session, unless you’re just getting hired to follow them around, but if you’re giving prompts to get the candid look you aren’t going to get a candid look. You can give them something unrelated to do, but again that’s going to depend on the constraints of the location and time allotted for the shoot.