If they can't be worn without a long top that covers the child's bottom then I don't think they quite meet the definition of "leggings" or "trousers". In fact, these are very thick tights. I returned them as they were not what we were looking for and my daughter did not find them comfortable. However, if you are looking for tights, they are very thick and I think they will be very warm.
Girls' Clothing and Leggings - Govc Winter Fleece Velvet Leggings
49 Review
Spotted Zebra Girl's Disney Star Wars Marvel Frozen Princess Leggings: Unleash the Fantasy!
46 Review
Adorable and comfortable Splendid girls' leggings for kids and babies
80 Review
Kid Nation Unisex Heather Charcoal Girls' Clothing for Leggings
52 Review