I’m afraid that I can’t comment on the suitability of the top bunks for children as I’ve never had any of either (!) – but I would definitely recommend a diesel version of these vehicles. They are always more reliable and economical – especially on short runs.
I’ve had both of the petrol types and the diesel version of the T25 and the latter was a far superior vehicle I always found. Spares are easily obtained for the VW too as it’s a classic design – the diesel shares its engine with the Golf. The Talbot is a bit physically larger it’s true, but not much – and feels a more modern vehicle to drive. Both types rust an awful lot everywhere though (as does the T4 actually) so beware of this in a potential purchase.
Hope this helps