It's safe, but the trail leading up to the falls is completely exposed to the elements, (i.e. full sun in the summer, wind and cold in fall-winter), so you'll need to protect your child with sunscreen, hat, and good shoes if you choose to put your child down as the walking surface is uneven and uphill 1/4 mile to the base of falls. Water and some snacs is also good to carry. The same applies to protection from the wind and cold if going in the fall or winter.

Really depends on what your comfortable in when hiking.

It all depends on how fit your kids are. This is a fairly easy hike, compared to most others nearby. If they are hesitant, I would say just try it and distract them with the views, a bit of history, the wildflowers, etc. Challenge them! Kids need hikes like this.

I haven't been up there recently, so can't answer about conditions. I do know that "the Big Melt" is on: with recent higher temperatures the snow is coming off the mountains quickly. So the falls will be magnificent from any distance. As to your friends "ability", it really depends on their aerobic capacity. Are they sedentary or do they do laps at the pool often? People adjust to the altitude much faster if they do hard exercise frequently at home. Why not just go to the trailhead and start up, taking your time, and see how they do? It's a marvelous hike no matter what.

You can absolutely drive to the base of the hike. We drove an Acura sedan to the base then hiked the falls. Anything past the parking area, I'd agree that you'd need a 4x4 with higher clearance