Stream SD version of HD purchases on iPad/iPod

Question

My family owns an iPod Touch 5, iPad Mini, and an iPad 3.  We have a 12GB/month data-only wifi hotspot plan with Sprint.  My two daughters are often watching videos on the iDevices while we're on longer car trips or out running errands and such.  My older daughter usually watches shows on Netflix.  Netflix usually isn't a concern with streaming content because I can easily adjust our Netflix account settings to stream at a lower bitrate.  Most of the shows she watches are SD rather than HD anyway.
My younger daughter, however, prefers all the Disney princess movies.  We have purchased many of them through iTunes.  We usually keep about three movies downloaded onto the iPad so she can watch them whenever she wants regardless of whether we have wifi or internet service.  We always purchase the HD version of every movie we buy on iTunes because when we are watching movies at home on the big-screen HDTV (and wired broadband internet), we want the HD version of the movie. 
Unfortunately, on one recent road trip, my younger daughter didn't want to watch the movies that were already downloaded on the iPad.  So, not thinking any better, we allowed her to stream movies of her choice from our iTunes purchases.  Lo and behold, because all the movies are HD versions, we chewed through almost our entire month's allotment of data in that one day.  By the time we noticed, we only had 1.8GB of our 12GB plan left, and we still had 25 days left in the month until our usage was renewed!!!!!!
So, my question is this.  Even though we buy the HD version of every movie through iTunes, is there any way to stream only the SD version when watching them on the iPod or iPads?  The iDevices can't display the full HD resolution anyway.  So, it makes no sense if we can only stream the HD version.  Is there some setting on the iDevices where I can tell them to stream at a lower bitrate?  Is there any way to chose the SD version of a movie when streaming on an iDevice even though we only purchased the HD version?
Please help!  This is going to cost us loads in overage charges if we can't figure this out.

Answer 1

Having the ability to stream SD versions of movies would also be helpful with buffering issues.  My daughter's movies are constantly starting and stopping and buffering because it is trying to stream an HD movie.  Streaming the SD version would prevent this.