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.