Doors open means you will be able to get in and if you want, find your seats, or find a spot in the standing area.
There will be stalls selling food, drink & merchandise in the venue, but also around and outside.
We always park one stop away from Wembley Park on the jubilee line (such as Kingsbury ) and have always been lucky to find street parking. Or you can hire a local driveway to park in. There is an official website / app for this.
I'd advise against parking nearby in all honesty- the crowds as they leave take hours to dwindle down, so you can end up queuing and waiting. But others might disagree.
Also, I always leave during the last 5 minutes to avoid the hell that is leaving alongside thousands and thousands of others, but your kids might see that as completely wrecking the experience...
You can't take anything in with you, the security is great but only take necessary items.
Buying food / drink will cost.
There are loads of toilets and queues go down quite quickly.
Wembley Park tube station normally closes its gates to let the platforms clear, then reopens until the crowds are through- another reason I like to run there before all of that.