Did you try an index on (type, ts desc) ? I don't have much else to add at this point, but maybe after posting some more server and table (parent and child) details someone will have an answer for you.
No, this is exactly what I was missing. I had forgotten the default index order is useless for a descending lookup like this: I made the change and the performance is 3000x better (the plan's using the index now). Thanks for all your help.
