I'm assuming that you have the vertical nipples if you are using the PVC pipe waterer. I've found that they tend to drip more than the horizontal ones (yes, I use both - vertical for chicks and horizontal for the main waterer) and that little drop on the end is what starts the freezing on the end.
I don't usually have any issues with the horizontal nipples freezing until we hit somewhere colder than 17 below zero. Then even they freeze.It's not the water in the bucket that freezes - the heater keeps that from happening - just the nipples. So while the coffee is brewing in the morning and we go out to do morning chores, we take a heat gun out with us. Just a few seconds with the heat gun and we're back in business. Sounds like a pain in the hiney, but I'd still rather spend a few minutes out there with the heat gun than haul water out there several times a day when it's that cold!
As you can see, we actually got icicles going from the nipple to the ground. In this shot the nipple part has been thawed and he was ready to attack the icicle itself. Residual water in the little cup part would freeze, and as it froze it would expand. That pushed the little metal part inward, and released more water which also froze. <sigh>
This year we've changed things up a bit. We got a different bucket and moved the nipples up so they weren't so low. So far so good, and we've had a few nights of sub-zero temps. I know a lot of other folks are using a system similar to yours with the vertical nipples so I'm hoping they will chime in here as their experience more closely matches yours. But in the meantime, you might pick up an inexpensive heat gun. It takes just seconds to free up each nipple and because they are using it throughout the day you should be okay. It's the overnight hours when they aren't keeping the water moving that tends to be the problem time.