I organise open days for a science department in a northern university.
Very much agree that these are primarily a way to get a feel for the university in question. Can your DC see themselves there? Do the facilities / accommodation / sports / societies seem attractive? It's just as much about what its like to live there as it is to study there (sounds obvious but so often forgotten).
Once your dc has registered for the open day, they should get a schedule for the day. You can drop into sessions that suit you, but some may need prebooking. Popular ones about finance, applying, accommodation will probably be run a few times during the day.
Check where / when the department talks / demos will be held, so that you have ample time to get from one to the other. If it's not a campus university, it could be further than you think!
Apart from that, enjoy it!! Staff and students are incredibly keen to show off what they offer. Feel free to ask tons of questions, however simple you fear your question may be. We're there to help. 