I seem to recall that you can authorize a kid's MB to have charging privileges but with a lower daily limit than your MB - is that correct? Is there some way to do this ahead of time or do I have to go to the hotel front desk?
No, you can't set a limit. But you can review your charges daily if you ask the front desk for a copy. We give our kids privileges and haven't had an issue yet!
You can check every day, I have had my daughter given privileges since she was 8 (now 17) with no issues, in her case I have even gotten her a Disney Visa Card (which makes it super easy to check every night)