Ok so the college age kids are off at school but how far away and are they in residence? If any has their own off-campus apartment and it's not a 20 hour drive to school, it wouldn't hurt to at least ask them if they would mind taking DD in for the week. When I was in undergrad, I probably wouldn't have been completely thrilled to host a sibling for a week, but I would have sucked it up and done it because I could. 13 should be old enough to sit through lectures responsibly enough, especially if they were anything like mine with 1200 people in them... the prof would never notice. Chem lab is a little tougher for tag alongs so I guess major would be a significant factor!
Again, depending on distance, if you have multiple kids in college you could ask them to timeshare babysitting by skipping some school. If one kid has a light first half of the week then that kid can stay home with DD until Wednesday, when the other kid takes over. I never bombed a class by missing a single lecture for a class. There are some courses that absolutely depend on being at every single lecture but, I'm three degrees in now, and I find them to be the exception rather than the rule.
It never hurts to ask. At the very least, you'll find out it's not an option and your DH is truly out of luck on the conference.