It's probably because the dogs are always really young. Children can seduce other children, right?
This was my hunch as well; LCS uses a pretty standard three-tier age of consent structure. Nobody under 11 years old can be targeted by or make a pickup line attempt. 11-15 year olds can only make or target pickup lines if they're both within 5 years of one another. 16+ is open season if both are in that range.
However, after reviewing the code, the real reason is that the age restrictions are entirely bypassed when either person is a dog, tank, or genetic monster. This is normally correct behavior (since it uses other restrictions, like humorous rejections), but it is not working as intended if it's allowing children to drop pickup lines.
Edit: The title of this thread creeps me out, since it sounds like a bug with seducing children. The content is pretty reasonable though, since the bug is that a kid can drop a pickup line at all, and even then it's guaranteed to be rejected. For all the LCS is free love, I've made efforts to keep the seduction mechanic from being used in ways that are creepy and dark.