heat. if they are worn on your chest you could skip a layer since your body heat and jacket can help keep them warm. also, make sure that you remove them from the carrier every so often to allow them to move freely and get good circulation going. if your child is walking on their own adjust their layers for how vigorous the activity will be.
try to buy quality name brand layers. the old saying is true “you get what you pay for” when buying winter gear. if you buy cheap character boots from a department store they are made with cheap materials and will likely leak and be cold. 