I bought this for my 4 year old as a gift to celebrate him starting school. He often asks me what words mean and so I wanted to get him used to the idea of using a dictionary to understand words.
I looked at a number of 'first dictionaries'. Some had a picture for every word which was nice, but they inevitably ended up covering only 100-300 words. Others were too complex (text too small, very few pictures). When I found the Usborne First Dictionary, I was worried that the quality of explanation may not be up to the quality of an Oxford or Collins Dictionary. However, every word my son has searched for has been well explained, brief and concise. There are also lots of pictures to hold his interest. The dictionary includes thousands of words - and almost every word we've searched for has been in there, leaving him satisfied, not disappointed.
I think this is a good all rounder, that will hold his interest for a few years, as he starts to be able to read the descriptions himself.