A few -10s are going together in the KC Metro area (I see you're in OP). While tight, you could make a 3-wide rear bench seat for the back (Cirrus does it....). The 5 year old would fit in the middle OK there for the time being, assuming small bodied kids. Given the age of your oldest, this might not be a problem for too long....
Or if purchasing a flying aircraft and extended family trips will be needed for 5 people + bags, then another aircraft (Cherokee Six, A36, 210, etc), might be more appropriate.
RV-10 Emp/Tailcone - Complete, QB Wings - Complete, Fuse - Building, Finish Kit - May '21
Never judge a man by his trim tabs.