So its acquired immunity and the cells responsible are the B lymphocytes that produces antibodies against the virus. Memory is keep since first encounter of the virus by the immune system by way of the antibodies that was initially produced. The antibodies are there already, which makes it fast to act upon when expose again.
Their age plays a role. Younger kids might be a lot more excited than older kids. How prepared they were before the new baby came also plays a role in how they will react.
