With an 8:15, you can enter the park a little before 8. There will be a line of people waiting, if you want to be among the first in, be through security and on line by 7:30. Seating will be first come first served, regardless of ADR time.
After they let you in, one adult take the kid to sign up for Jedi training (it's just past H&V). The other can check in at H&V, and get your table (if not back yet from Jedi sign up). The participating child MUST be present at the sign up.