A child that has learnt the words cat and dog by word recognition will not be able to read cog or tag. A child that has learnt the phonics for c,a,t,d,o,g can read cat, dog,cog, tag, got, god, cod, cot
Unless the child in question can actually read cog, tag, cot, tot and various other common words!
It is possible, if the child is a bit simple-minded (or hasn't been taught to spell, yet) that she can't read:
sog, gog, kog, vog, tob
and so on, because she hasn't worked out how to string random letters together in order to make pointless words. I'll grant you. But when she learns the power of spelling, even a look and say child can make up pointless wrds!