We always used pockets for under trousers, so BB or WNOS normally. Under dungarees or jogger bottoms. Leggings are handy for girls then once it gets warmer it's all skirts and things so I guess it is a bit easier then. I like bamboo interlock and bamboo velour trousers, minkee too, though maybe they are quite girly.
Anything with a bit of stretch generally works though. Synthetic fleece is good too, but I'm a bit funny about using synthetics, except for jumpers and Snugglebots. Otherwise I like natural fleeces for warmer trousers.