So, is it O size trains?  But running on narrow gauge track?

I'm interested in getting a train set to go around our tree, so that my 4 year old and 2 year old can enjoy a train during the holidays.  We don't plan on using it beyond Christmas time.  I have never purchased, or used, a model train set before.  I have been reading about HO and O.  The consensus seems to be to get an O set (or at least something bigger).  I saw this set on Bachman's site and was wondering if anyone had any suggestions, as I'm not familiar with On30 scale: http://www.bachmanntrains.com/home-usa/products.php?act=viewProd&productId=3188
Thanks!

Hi,
I am new to train models.  I plan on getting a set for running around the Christmas tree this year for my family to enjoy.  I have two young children, who will not be using it without adult supervision.  And the set will only be set up during the holidays and won't be used year round (til the kids get older).  What scale do people recommend getting? Any specific Bachman sets?
thanks for your help!!