To solve the problem, we can consider A and B as a single unit. This gives us three units to arrange - AB, C, and D.

Now, we can arrange these three units in 3! ways = 6 ways.

Within the AB unit, there are two ways to arrange A and B. Therefore, the total number of ways to arrange the four children such that A and B are always together is 6 x 2 = 12.