Briefly, one or a few mutations are responsible for both the child-like traits that are selected for sexually as well as other child-like traits such as lactose tolerance. So selecting for the former also results in the latter, because the traits are genetically linked.
Typically neoteny is achieved by mutations that alter developmental timing, and it's easy to imagine that such a mutation could also result in sustained lactase production past the "normal" age.
As for the question of why all lactose-tolerant people aren't visually identifiable, keep in mind that there may not be just one variant of the lactose-tolerance mutation, and perhaps not every variant has complete penetrance. For example, perhaps one gene variant confers a 50% probability of lactose tolerance, but always confers a particular attractive trait.
Typically neoteny is achieved by mutations that alter developmental timing, and it's easy to imagine that such a mutation could also result in sustained lactase production past the "normal" age.
As for the question of why all lactose-tolerant people aren't visually identifiable, keep in mind that there may not be just one variant of the lactose-tolerance mutation, and perhaps not every variant has complete penetrance. For example, perhaps one gene variant confers a 50% probability of lactose tolerance, but always confers a particular attractive trait.