I'm waiting for swatches to come in, but in the meantime I've been searching for patterns. Not much in the way of commercial patterns billed as "Pattern for guys to make their own super-spiffy hardcore technical base layers and look cool doing it". But there are plenty of pajama patterns, shirt patterns, etc.
So I pulled open my drawer of "super-spiffy hardcore technical base layers" and threw them on the floor. Turns out they're either very simple raglan or very simple t-shirt cuts. The only place with much complexity is the collar and zips on some. I want to make hooded base layers but I don't have any, so I pulled out a couple of lightweight balaclavas. Also amazingly simple- the simplest is just two matching pieces of fabric. I pulled some of the kids' pajama bottoms out and they're simple too- just one piece of fabric for each leg- inseam only, no outseam.
I've decided to skip the pre-made pattern and find a garment that fits each kid, then copy that. Raglan sleeves appear easiest. I think that a balaclava hood will actually be easier than a good collar. Bottoms look incredibly easy.
The goal here is to save money and get high-performing stuff that fits the kids. By my calculations I can make a full baselayer pair for both kids with about 2.5 yards of fabric. That's about $5 per piece. Worst case is I'm out a few bucks for fabric.
Wish me luck, and please tell tales of success (or lessons learned the hard way) on similar projects.