Erm...could be good but not sure Dr Who is particularly big amongst my son's friends (he's 6)...but then it could be in other schools, who knows. I wonder if it might be bigger amongst slightly older kids as not every 6yo or 5 going on 6yo will be up or allowed to watch Dr Who? But I don't know tbh....anything with aliens and monsters is pretty popular with my DS so maybe it'd go down a treat!
Ninjago lego (newish ninja themed lego) was my son's number 1 favourite present for his 6th birthday this week. There are some inexpensive mini kits and big supermarkets and Argos will have it. K'nex construction kits also good but not the "mini" version - he struggles with that. Lots of boys in his class (and him) are into natural history - especially bugs, snakes, wild animals etc.
My son is turning 6 soon, and loves listening to Dr Who on Radio 4 Extra, so I think the audiobook could be a great idea. There also seemed to be a lot of kids in his Reception class last year who were familiar with Dr Who (although we didn't let DS watch it then).