Stretching- maybe a couple simplified versions of sidewinders drills, total of about 15 minutes for that,
For driving practice, 2 lines of 10 people, starting at one side, throw the disc, run to the other side and pick up their disc on the way, second line repeats, go back and forth. Make smaller groups of 5 and 4 lines for more individual attention.
Putt like a relay race- divide them evenly among your baskets- front person putts, grabs disc, and runs to the back.
I haven't taught disc golf to kids in groups before- just my 8 year old- but I am an elementary school teacher, and the best thing you can do is keep them moving and changing things up. Holding the younger ones attention for even 5 minutes can be challenging sometimes.
I would advise heavily against them throwing toward anyone else- like in pro clinics when everyone putts in a circle. There are gonna be some kids who throw hard on purpose and someone it gonna get hurt.