We've only ever done rec soccer. They have never worn pants. Always athletic shorts. You don't have to buy any specific brand, just a cheap pair would be fine. My boys always wore base layers under if it was really that cold (and here, it would be more because it was drizzly and cool than truly COLD most of the time). We already have those because of skiing and hockey though. Leggings and a long sleeve shirt would probably work for a girl. Depends on how cold she really gets.
Mama to my boys (04,07,11)