When the Bedford 4-Ton truck is tipped over the cliff, the model that's initially pushed with a digger is the mid 1970's model previously driven into the scene, but the one that tumbles down the hill is a much older (late 1950's/early 1960's) model with different style of cab.
When the truck is seen tumbling down the cliff you can see there is nothing underneath the hood (or bonnet) where the engine should be.
The child is kidnapped and placed in the back seat of a taxi. The back door is locked. He could have climbed out through the open window.
In the WW2 fantasy scene Niven asks for a bazooka but he is given a British Army PIAT anti tank spigot mortar.