There are many techniques to keep your little one warm in a stroller in winter. The best way is to use a stroller footmuff since it is designed for this purpose exactly, doesn't interfere with five-point harness systems in any way, and has a waterproof outer shell. An alternative way is to use a blanket or to dress your child in many layers. For covers and blankets, use materials with a high-temperature rating, that is, materials that retain heat.