The TB9 was produced as a trainer, not a touring a/c. Your kids will grow during the time you own the share in the plane and so you won't be able to go far with four up with some bags. In a group, this leaves the problem of how much fuel is in the tanks when you turn up at the airport en famille.
I had a share in a TB10 which was very capable but 90% of my trips were 2 or 3 up. You really need to fly a TB to see what the benefits are ("modern" interior design, great visibility, rear pax sit higher and get good vis out front etc).
A summary of the TB series:
TB9 Tampico O-320 160bhp - fixed gear & prop
TB10 Tobago O-360 180bhp - fixed gear, variable prop
TB20 Trinidad IO-540 250bhp - retractable gear, variable prop
TB21 - TIO-540 250bhp - retractable gear, variable prop & turbo normaliser (sea level to FL200)
I'd view the TB9 as a 150 but with seats in the back and look out for a TB10 share if you like the type. There are quite a few around.