Recently I came across the image above of small bunk beds in the shared bedroom of 4-year-old twins and realized I'd never seen mini bunks like this. They are the same size as a toddler bed (i.e. the size of a crib mattress) and offer a stop-gap solution for parents looking to eke out another year or two in their current space.
First, a word about safety: Although these beds are sized for toddlers—i.e. toddler-size or crib-size mattress—they are not intended for very young children. Bunk and loft beds inherently carry safety risks from falls and entrapment. You know best at what age a bunk is appropriate for your kid. My own son, for example, moved to a low loft bed when he was 4.5. He is almost 6 now and still fits in a toddler-size bed, but many kids his age would already be too big.
Over the years readers have weighed in on the pros and cons of toddler size beds which have a short shelf life and are outgrown in a few years, but I know several families in my neighborhood whose kids share a very small bedroom and they're trying to find solutions to make it work for another year or two. Perhaps these smaller bunk and loft beds could work for them temporarily.
Here are some examples I found if this is something you are exploring. Some of these are for sale and others are DIY projects.