How do we charge a small 12V battery from a large 12V battery bank
To charge a lead acid battery you need more than 12V. Any charger would be a combination of a DC-DC boost converter and some circuitry to control the charge current.

Probably the easiest off-the-shelf solution which you could piece together in a hardware store would be a DC to AC inverter coupled with any standard lead acid charger. It wouldn't be as efficient as a DC-DC converter charger, but would be a lot less specialised. There can be issues with cheap modified square wave inverters and some devices - I'm not sure if that would be an issue here.

There is devices which will do what you want. This company seem to have a whole range of them. (I've never had any dealings with them, they were just the first result when I googled "DC DC lead acid charger".

Finally, you could build something yourself. DC DC boost converters are fairly ubiquitous and a lead acid battery charging IC like the uc3906 is available in a DIP form factor and has a fairly low part count requirement. It would require some design work and knowledge of electronics though, so if you're looking for something as simple and rugged as possible this isn't the ideal approach.