To ensure children A and B are always together, treat them as a single unit (AB). Now, we have three units to arrange: (AB), C, and D. These three units can be arranged in 3! (3 factorial) ways, which is 3 * 2 * 1 = 6 ways. Within the (AB) unit, A and B can swap positions (AB or BA), providing 2! (2 factorial) or 2 ways for their internal arrangement. Therefore, the total number of ways is the product of the arrangements of the units and the internal arrangements: 6 * 2 = 12.