The main problems with 'Warriors of the Deep' are the Silurians/Sea Devils are reduced rather to monster-of-the-week - without the moral centre that made them interesting in 'The Cave Monsters' and that everything is so brightly lit. There's a lot of interesting stuff going on in 'Warriors of the Deep' that chimed well with the early 80s - but things like the Myrca and the generally unatmospheric direction detract from the ideas in the script.

My boss's 6 year old boy was apparently sat down in front of an old Tom Baker episode recently and immediately started complaining that the monster looked fake.

Depends on the story really - some worked better than others - which still is the case today. Personally speaking I don't think the Slitheen, for example, are a particularly good prosthetic and in Aliens of London where it switches between prosthetic and CGI it looks especially naff. I reckon that a young fan of today would like most colour Dalek stories (though probably not Genesis or Ressurection - way too dark for a 6 year old) and wouldn't have a problem with the effects, I would say the problem with a lot of classic Who for a young viewer would be the pacing...