From an academic: the main thing is, let your DC do the talking & asking.
Rehearse with your DC the questions they might ask of the academic teaching staff, while you look on from a distance. Better still, leave them to it, and attend a talk on finances. Let your DC take the lead. Don't step in - if they forget to ask something they can go back to talk to academic staff later, or ask at the talk, or ask student helpers.
Make sure your DC has some specific things they want to ask about - my place does one of the earlier Open Days of the season, and I get the question "What's your degree like?" So I then have to spend a bit of time trying to wheedle out of a nervous 17 year old what it is they're interested in about the programme I teach in. So see if you can help them work that out at ho,e, so when they meet a real ive lecturer or current student, they have some specifics to ask about or talk about. I teach in the humanities, so our programme s pretty broad. If they have a particular ambition, or interest, then they can ask "I'm really interested in learning ore about X - what sorts of opportunities will I have in this degree?" Your DC will get a more specific and hopefully helpful answer.
Also rehearse what sort of topics your DC might ask the student ambassadors - they know about.
You are best placed to go to the parents' talk, talks on finances and accommodation, and if your DC needs extra help (learning disabilities etc) go to the student support talk. All of these so you can support your DC.
Things not to do -
ask about face to face teaching hours. It's not really a particularly useful question. My students have a lot of reading to do; STEMM students have lots of labs. Both are necessary for progress & learning. I suppose I could do a "reading lab" two days a week with my students, and that would count as face to face hours - but it would be silly. So the number of face to face hours doesn't really tell you much.
A much better question is to ask about what forms of teaching and assessment they'll encounter in the degree course. eg lectures, seminars, individual tutorials, group work, independent work, placements, etc etc etc.
Another thing not to do is make fun of or in some way neg the subject your DC has chosen (the number of engineer fathers who try to put down my subject as "easy" ...). You & your DC are talking to highly trained and very hard-working academics, who - simply by having university jobs in the current climate - are at the top of the game. Don't treat them disrespectfully! You would be surprised how often this comes from parents. If you have misgivings about the degree choice of your DC, hash that out with them, rather than use academic as your go-between in a debate with your DC.