Fleece is my fabric of choice for layering my kids at night camping. Also great for its awesome wicking properties - it will keep a person warm, even if it gets wet. But in my experience you get what you pay for it. Cheaper fleece pajamas are great for winter nights at home, or maybe as a base layer. But I've noticed they have about 1 year, maybe less before they begin to pill, degrade and thin. For example, I bought my daughter a North Face fleece on clearance when she was 2 1/2. I bought it big and rolled the sleeves. She still wears it - and I still roll the sleeves sometimes. There has been no pilling. No thinning of the fabric. And it is so warm I worry more about her overheating and sweating, than getting cold. In the same amount of time we've had this fleece, she has been through numerous Old Navy fleeces that I hesitate to even pass on to her younger sister.
So the bottom line? When I want my kids warm, I invest in their fleece. I scour my favorite outdoor sites for sales. And it has served me well. Check the state of their fleece before you head out camping, and know the thinner it is, the more air it will let through. And you don't have to spend a ton! All our kids have REI fleece jackets and have for years - on sale most of them cost only $9.