When? The answer would be different once winter arrives at CL, which could happen in October.
Aloha Nui & Howzit, MalieAloha:
I-5 is very safe, easier to drive than H-1. Like oregonpoppa wrote, time of year makes a difference. Crater Lake averages 43 feet of snow per year, most of it from December through May. The roads around the lake close for the season with the first big October snowstorm, or on November 1, whichever comes first, and stay closed until Mid May-Late June for the West Rim Drive and North Entrance and for the East Rim Drive until mid June to late July. Google Crater Lake National Park for more info. That said, southern Oregon has lots of great reasons to visit anytime. Hele mai!
My thoughts are that the freeway drive is the simplest part (and least worrisome) of the equation.
But mountain driving to get from I-5 to CL, that could be challenging depending on weather. Not to be done at dusk or at night unless there's a very experienced driver (issues: weather conditions, animals on road, etc) and a well-prepared, dependable vehicle this time of year. Getting a flat tire out on a mountain road where there's no cell service sucks.
CL "for a few days" means lodging. Already booked? The lodge books out a year in advance. Planning to camp? if so, same question about reservations. If camping, this also means do they have the right seasonal equipment and clothing.
I regret, MaileAloha, that I have expanded your Potential Worry List. But these are things your daughter may not have considered. I remember when mine were that age, and they felt invincible and immortal! However, all in all, I would not be overly worried about a trip like this, again as long as they are prepared. CL is a family destination, a national park, not the wilderness.