If my child loved horses and was interested, I would go. The theme is very specific and not everyone loves horses and would be interested in that type of party.
That type of drive is nothing around here in the Dallas area in order to get to downtown, airport, special arena, state fair, trade center. Many places are within 30 minutes in good traffic. The hold up here is the unknown traffic issues. Weekends are usually not bad and everyone knows to stay off the expressways between 3 and 7pm. Drive time does not bother me because it is routine.
Is this to be a drop off party for the guests? If so, parents may feel stuck in an area with nothing to do for a couple of hours. I can see where some parents would not make the drive if they have nothing to do or other children with activities around that time.
An option could be to carpool or rent a limo which holds about 10 people and provide transportation.