I would be more concerned with rocks, barnacles, logs, hooks, crab traps, corners of the battery, etc puncturing an inflatable than I would the kid getting odd bump or bruise from a tin boat.
Also, in 10 years, the tin boat will worth either the same or more than when you buy it. The inflatable will be deteriorating and will depreciate.
Don't get me wrong, I have an 8 foot zodiac type of inflatable that I use in place of a pontoon, I love it for that purpose. It also fits in my MX5. So when I am fishing solo, I don't have to take my truck. That said, I also have a 14' tinny with a 15HP Merc which gets used 98% of the time.
I beach it anywhere, run in log infested waters, bounce of barnacle covered rocks, spill the odd bit of gas etc. I just generally don't have be extra cautious like I do in my inflatable. 50% of my fishing is in estuaries and the Fraser, the other 50% in lakes. If all I did was hit lakes, I might, MIGHT consider running just a larger inflatable. But for me, the tinny is just a better option.