Also in Catman Comics I believe both the Deacon & the Reckoner had kid assistants. BTW, both characters were more like the Spirit than Batman.
Over in Novelty's Target Comics, the Target had the Targeteers. They were two adult men, brothers, IIRC.
The Clock, who appeared in both Dell & Quality books, was assisted by Butch. Butch was a girl who dressed and lived like a slum tough guy.
At DC Simon & Kirby's Newsboy Legion tween / teens were regularly protected by the masked Guardian. Of course you won't find those stories here. (sigh)
In in the UK/Aussie/NZ section of DCM there are three issues of John Dixon's Australian version of Catman, who had the boy Kit as a costumed sidekick.