I had a "fun" session with all four of my kids (ages 1,2,9,13) the other day although it wasn't perfect, I didn't run home and drink an entire bottle of wine afterwards either.
Some things that helped it go well included:
*letting the big kids dress themselves (with some suggestions/guidelines)
*snacks/treats (smores flavored goldfish!)
*wipes (to clean snacks off faces)
*privacy (no cars driving by or other people around -- critical with my older ones)
*safe space to run around and play (critical for my 2 year old)
*LOW expectations (I was hoping to get one decent shot of each kid)
*no group shots (too stressful by myself when the littles are trying to squirm and run away)
*LOTS of praise and letting them pose themselves
*working quickly (we were there less than 45 minutes)
I wished I was able to use my reflector, but had 1 yr old on hip part of the time and no free time or hands to position it. As soon as I realized "I" was getting frustrated and stressed out with the kids, we piled into the car and went home and I didn't have to deal with momtographer induced meltdowns.