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.