The folding front seat mattress idea probably traces back to this well-known Roadtreking blog post
because I believe that guy was the first to develop the idea.
Your best course of action would probably depend on how many kids and what ages and what your budget is. There are other Class B's on the market that have at least one extra bed built in as OEM - my husband and I looked at them quite closely because we have a daughter. But we ended up concluding she shouldn't be a deciding factor in our purchase decision because she was a teenager and her interest in RV travel approached negative infinity (and still does).
The DIY Sprinter camper van community has a lot of info and ideas on adding beds. Much of it can be found on Instagram, FWIW. The most common way in which space efficiency is gained over a stock model such as the Interstate is to raise up the main bed such that there is space underneath, either for storage or sometimes for other beds.