There's two ways someone can be promoted to moderator on a site, and please keep in mind that while child meta sites are linked to a parent, they are (for all intents and purposes) independent sites (same code, different setup).
The first way is appointed, which creates a user history record, which is what fires the constable badge. If you're appointed on the main site, you're automatically appointed on the child meta.
The second way to promote you is 'by winning an election', and this reason is tied to the elections the main site had. Since there's no election on the meta site, you get promoted for winning on the main, and appointed on the child meta. No one is ever elected to be a moderator on a child meta site, because the child meta site has an empty election database table.
I'll see what we can do to fix this, possibly broaden the award query to see if you have the badge on the parent and trigger it on the child meta, but I have to see what kind of side effects that sorcery might have before we do it.