Can I set up an additional log in on my account, so that someone else can ONLY change a route to number at individual number level?

I am afraid that this is not possible unless you have a Partner Package with us, as one of the benefits of these packages is to provide Number Admin which would allow you to give your customer individual access, without being able to view any costs associated with their number. 

If you are willing to allow them full access to your account, you can have a secondary log in which will allow them to make changes to all the numbers on your account and view costs relating to your whole account.