I'm of the opinion that the choice to blanket is a personal decision. Some people will tell you that blanketing is terrible and some will tell you that you should never ever blanket.
Basically, it's something that should be decided after you understand all the factors and weigh them in your mind to determine the right choice for you and your horses.
I'm perfectly fine with blanketing. I think young chubby horses with shelter most of the time don't need to be blanketed (I don't know if your horses fall into that category). However, I also think it's a jerk move to put a saddle on a wet horse (due to the fact that you really can't clean a wet horse with brushes) so horses that are to be ridden should be dry before they are ridden and the easiest way to do that is with a blanket. I have ridden a very wet horse many times and that horse has survived, I just felt like a jerk for riding a dirty, wet horse.
I think older horses should be blanketed as a general rule. Their internal temperature monitoring thing is getting a little creaky at their age, and I think it's just more respectful to help them keep themselves warm.
Personally, I think that if it makes you feel better to blanket, your horses aren't going to be harmed by being blanketed. As long as you follow a blanketing chart (those ones that tell you what blanket weight for what temperature), your horses will be just fine. Just don't go sticking a lightweight sheet on your horse in 30 degree weather and expect that to warm your horse. It doesn't work that way.
That being said, I've seem tons of people recommending these blankets as great things (don't have one myself, yet) but I'm not sure if they come in a 64... StormShield® CLASSIC EURO Contour Collar Turnouts in Euro Fit at Schneider Saddlery
Here's a page that seems to have quite a few pony blankets (aka, size 64): Junior / Pony Blankets at Schneider Saddlery
Of those, this one seemed to be the best priced but I'm not sure why: http://www.sstack.com/horse-blankets...nior-turnouts/