Unless they added the feature to the Grand Tour, I'm not aware of any Airstream Interstate that has provision for child safety seats. Federal Motor Vehicle Safety Standard (FMVSS) 225 "Child Restraint Anchorage Systems" only applies to vehicles with a GVWR of 10,000 pounds or less. A Sprinter 3500, the base vehicle for an Airstream Interstate since about 2011, has a GVWR of 11,030 pounds and is therefore exempt.
You should be able to find child safety seat systems for use in aircraft that will attach to just a lap belt, without needing the standard anchorages found in passenger vehicles under 10,000 pounds GVWR. That's about the best you're going to get considering that Interstates don't have a child restraint anchorage system.
Engineering: Finding complex solutions to simple problems you didn't even know you had.