As the photos show, child booster seats can fit quite comfortably on the Suzuki Jimny's rear seat.
Simply follow the installation instructions on your particular child car seat - some have rear anchor belts (similar to the infant car seat systems) and some don't.
The photo below shoes a 2 year old in the booster seat and the infant car seat installed next to him.
Now, comfort is another thing altogether!
You may be wondering about the leg room available, and how far forward you may have to go in the front seats. No worries, with a 175cm adult in the front seat, slightly forward than normal, there is approximately 12cm of rear leg room for a 2 year old to kick the back of you!
If you have a toddler in the infant car seat (left) and a child (4 year old pictured on the right) in the booster car seat, there is still plenty of leg room for both in the back.
Note that the padding of an infant car seat will raise a child's legs so that there is less leg room, however the booster seat allows a child's legs to dangle down over the edge of the seat.
So yes, everyone can fit, but remember that space is a compromise in the Jimny. We recommend stopping on long drives to stretch the legs and refresh the spirit!
Also note that if you have both rear seats occupied with child car seats that have rear anchor belts, there VERY little room in the rear storage. You would be hoping to stuff items down into the footwells - and rules out any thought of taking a pram with you. If you need to take a pram and/or luggage, then consider the Jimny as best for a one child family.