That makes sense - the reason it's in the Law faculty is because it's close to the centre, nice big lecture rooms with good facilities for disabled access (IIRC), potential for parents to buy coffee nearby if the on-site cafes are open, etc.
You may want to get a map and visit the maths dept if there's time and if it's open, but at least you can see the architecture.
You'll be right next door to Selwyn College so can take a look round that very easily, and Robinson College is on the way to the maths dept.
You're also near to Clare College, Kings College and Trinity Hall. You won't need to pay tourist entry fees if you're visiting as a potential student. Not sure about visiting as a potential student's mother without student in tow!
Have a lovely time! I think the nicest shops are on the little back streets ringing out from the Market Square, which is almost all chain stores. Benet St, Bridge St, etc.
Grendelsmum parents can book but not participate. I've booked it for him and we are going on the train. Oddly enough the Maths lecture is at the Law faculty. I will have a wander round while he is there.
I'd imagine that they're well worth going to for an enthusiastic student - they've probably pulled out some good speakers who will appeal to the year 12 students, and it will give them a better idea of what undergraduate study would be like.
Cambridge is lovely, so you can spend 4 hours shopping and visiting quite happily.
As Boomting says, you might be able to squeeze in some quick visits to colleges, although the physics and maths departments are slightly out of the town centre in two locations about 10 mins apart.
On the other hand, it looked to me like the teacher needs to book the place, not the parent?
It's only one day, and it may be possible to combine it with a campus tour or similar, and the chance to look around the town more generally. That will mean that you'll have a better idea of whether or not it's worth visiting for the 'proper' open day later in the year.
My DS did a couple of these. Its was a 3 hour journey each way but as DD was at Cambridge uni this seemed worthwhile. DS found them very stimulating and it was all helpful on his uni applications! He's at Imperial now - its very challenging and I think it probably helped him in a general way to go to those lectures and realise that uni would indeed be a challenge. He was very proud of one part where he managed to follow a tricky series of things. If he's thinking of applying to Cambridge its also an opportunity to look round?