Would not have tryouts, you never know who will develop.
No specific times... the best at this age do not always translate to the best later in their "careers"
We are a club, practice two evenings per week plus Saturday afternoon - 90 minutes.
Warmup - All run 800, some a bit more; then 15 or more minutes of dynamic stretching.
Cool down not a strong point, usually parents arriving to pick up @ 8:30PM... but ideally another 800, some stretching.
We do three circuits early in the season, we do all events except discus and PV... so kids rotate 20ish minutes thru three events. After about 6 practices, allow them to choose their events and do 2 x 30 minute sessions - Mid-d, sprints, plyo, hurdles, jav, shot, LJ, HJ, turbo jav.
Found that kids LOVE the plyo, and there's a lot you can do there to make it fun, contests (who can hold plank longest, small team competitions).
Good luck with your season.