Clear as mud
Ok, so what you want is a combination seat (this is a forward facing seat that works as a harness and then eventually can be converted to a booster.)
You want a seat with a higher weight harness that eventually becomes a GOOD booster (there are many on the market that become bad boosters. Avoid them!)
Unfortunately, the terms 'slim' and 'combination seat' generally don't go hand in hand. However, the slimmest, and also coincidentally the cheapest, is the Evenflo Maestro. It will be outgrown both as a harness and a booster by around 6 years old (keep in mind that other good combination seats, the Graco Nautilus or Graco Argos, Britax Frontier 85 and Recaro Prosport will last much longer.)
And it's still going to be tight, unfortunately. If you intend to put them both outboard and have an adult squeeze between the two, keep in mind that the middle seating position needs to have a lap shoulder belt and adjustable head rest to keep that adult passenger safe (every adult needs to have the head rest adjusted to at least the tops of the ears).