Which training session to let go?

The run is probably easiest to fit into your schedule. I wouldn't worry too much about missing a training session here or there - all of us do. There's no need to be a slave to your plan... If you feel like the session you miss according to your plan is a key session, then include it in your training early next week in place of a "less key" session.

Honestly, there's not much you're going to do this weekend or early next week that's going to impact your race next weekend (unless you over-do and go into the race fatigued). Is the race next weekend an important race to you, or more of a fun, train-through kind of thing? If it's an important race, then I would say just fit in whichever workout you can do most easily this weekend, and just let the other one go. If it's not an important race, and you're using it for experience and/or to build for another race, then you can try to fit the session you miss this weekend in early next week.

Bottom line - don't sweat it.... Do the best you can, and enjoy the weekend with your family!!