In agreement to the first advice. Just politely let this Mother know that your show will last 30 min. Apologize for any inconvenience, as she may have planned around you.
A tip for next time (which I'm sure you are learning already, but just to articulate it...)
-Confirm the details of your gig before you go into it. This included cost, time, & expectations. If the client wants a longer show, let her know the extra cost, and possible difficulties that may be involved (like lack of attention spans).
-Also, just because you can't meet her expectations of time, don't lower your price. Remind her that the price you discussed earlier was for a thirty minute show. If she feels that is too much money, then you may lose that gig, but you will maintain your integrity as a performer, and it will give you more time to prepare your show for the next time. Don't sweat it.
-Make sure your routine is really well practiced. Don't add stuff last minute. -Take your time with each routine. (If each effect (hypothetically) only lasted 5 min, then you only need 6 routines. As your routines become polished, they can last much longer. -for example, the performance of Luke Jermays latest routine EI was around 25 min and there is really only 3 effects in the routine. )
-Also feel free to post your show routine here and get lots of feedback (positive and negative). There are a lot of smart minds here that can help contribute and tweak your routines and overall performance.
-and lastly, make sure next time, that you have your show FIRST, then sell it, so you don't run into this 'last minute panick' problem again.
Hope that helps some.
Good luck!
justin