To find the ratio of children to keychains, we first need to determine the total number of keychains.
There are 6 children, and each child has 2 keychains. Therefore, the total number of keychains is:
\[ 6 \text{ children} \times 2 \text{ keychains/child} = 12 \text{ keychains} \]
Now, we can express the ratio of children to keychains:
\[ \text{Ratio of children to keychains} = \frac{6 \text{ children}}{12 \text{ keychains}} = 6:12 \]
Thus, the correct response is:
6:12