Man alive (and alive is the key word!) this has got very aggressive over what presents to me (a long term SIII, County and Defender owner) as a very straightforward question.
And for the record, I'm a GP and as interested as anyone in keeping my family alive.
On the safety issue - this car has one of the best safety records imaginable in the UK, but not so good in Australia. The difference? they are slower on back roads over there. All this stuff about side intrusion etc - the things have a solid chassis with outriggers. The "safety" issue is that they are LESS flexible than a modern design. So don't hit anything at high speed. But with a 300TDI that's unlikely!
Now to the original Kid issues :-)
Seatbelts/seating: kids love the rear seats of the Defenders. They have good views even if the seats are slightly less than ideal. The TDI era seats are fine (improved over the Series/County seats) though the Puma seats are much better. I don't know re legroom - the mid row in the Defenders is less than you would think due to being able to seat 4 behind the "rear" seats. That said, I will fit in the rear seats behind a front seat set for me. I'm 5'10 or 178cm.
Aircon: While the AC is routine dissed, it's not that bad. The County AC on the other hand! The TDI era Defenders all have Australian made and installed "factory aftermarket" AC which if gassed and working will do the job. 40 degrees in Qld summer may defeat it - but engine cooling will get you then as well. I have a clear plastic sheet on the cargo barrier so I don't have to AC the load area. I don't know if it works :-). We recently had a trip through northern Qld and the NT. We did run the AC all the way dawn to dusk and things were never cold in the cab, but never hot either. I'm a little puzzled at the earlier SIII comment but then mine had a Tropical Roof - two roof layers with venting between, with controllable vents sucking air from the interior into the midzone space. This was the bees knees of non-AC venting, taking all the hot air out of the cab.
Heat gain through flat glass: don't make me laugh. The windows of the Defender are small and more vertical than most 4wds. They get less overall sun than most. The real heat gain is through the floor. And ALL glass has a SPF of around 50. There is no more risk of sunburn in a Defender than any other car.
Seatbelts: you will need a CRAB - a Child Restraint Accessory Bar. These come variously fitted - I installed mine across the back of the C pillars just above the rear wheel "arch". This means it attaches through steel (the car it came from the mount went through aluminium) and increases side intrusion resistance. For this reason I prefer the longer side-to-side versions - and they also have minimal intrusion into the cargo space unlike the floor mounted versions.
Side intrusion: As someone has mentioned, Rock Sliders will stiffen the side intrusion resistance. Anything above foot level is unnecessary as other vehicles don't come that high.
Rollover: Yep, Defenders are poor at this. So were all 4wds of the era, and all sedans until the 1990s. Manage this as you see fit. I personally think the chance of the issue is relatively low, but a rollcage or at the least a cargo barrier are good options. The plus side is that Defenders have little weight above the chassis level. They are less likely to roll than a stiff and strengthened modern 3 tonne 4wd. Just don't overload the roof.
The general idea is, as has been said, when driving you need to pay attention to your surroundings. You need to get familiar with the front and rear tracking, aim to anticipate what others are going to do (or not do) in a similar way to what a motorcyclist does. The regular incentive is that your brakes and motor are not that hot! The additional issue is that the passive safety is lower in a Defender than many vehicles - so keep your active safety at the highest level.
I've certainly put my money where my mouth is: A total of 3 Land Rover wagons (plus 2 utilities) through my 3 children's childhoods (still ongoing!) I did get rearended on the motorway once: no damage to the Defender, major damage to the little sportscar. I had 3 kids in the car then (2 not mine) and they all regard it as a "Tank" ever since.
