I like to use zoom with some reach for the kids on the playground. Nice picts without being in their face. I use my 55-250 for that and it's swell but a cheap lens. I'm thinking the 85mm might be ok for out and about with the kids but of course too tight in doors.
Nice lens on a full frame. My neighbor used it when trick or treating with us on her 5D and her pictures with the 85mm blew away mine with my T2i and 30mm F1.4. Amazing IQ that made the T2i with the sigma look like a point and shoot.
For sports though 85mm is very short. I shot one of my kids soccer games with the thrifty 50, that was way too short, and it's hard to imagine an extra 35mm would be enough.
I have a 60D and a 50mm f/1.4 and I think the quality of her shots are probably more so related to the higher ISO and less grain you get when you use a 5D (mk 1, 2, or 3). That is just an incredible low light camera.
Having a great time and visiting family are mutually exclusive.