Welcome!
What exactly were you expecting that you’re not seeing?
Your model represents a bit of a paradox (not an actual paradox, but a bit awkward). In the limit of large data, one might hope that the SD of child should approach 1 (the true SD) and the SD of parent should approach 0 (because the true mean should be estimated with infinite precision). But each of these SDs is tied to a single parameter (shared_sd), which forces a compromise that is influenced by the amount of observed data.