I'd be dubious of a dealer being able to provide accurate information like this. I would request they write to MB head office to ask for clarification, as it's an important decision. I'm sure you're not the first person to think of this issue so I would hope MB would have the answer somewhere.
It's very likely that children will be in the rear of a car so when designing rear airbags I would imagine this would have been taken into account.
Side impacts are possible the worst type you can have, so having more airbags would be a natural choice if safety is paramount.