"I find your lack of vocabulary disturbing...."
Lol.

I have to ask, what is the point behind using a "Junior" dictionary? I mean, what specific purpose does it serve that a regular dictionary does not? When I was a kid we just used a regular dictionary.
^Indeed, you have a good point here. I say an OED for every house!
Well, we have the OED and junior dictionaries (though not the OUP ones) in our house and I can tell you why both are good to have. The OED is the 2 volume
cinderblock err mighty Tome set with 4 pages printed on each page in tiny print* (and wouldn't you know that the children seem to have carried off the magnifying glass that came with it

). It is the dictionary of choice for meanings, roots and origins of words in the English language and it doesn't leave things out. When the older children (15 and 12) don't know what a word means they get referred to that.
Our youngest however, who is learning to read and with his Downs needs a bit more help, is very fond of his Dr. Suess Dictionary which has a
picture with each word and they are all words that a small to medium child can be expected to need/read/use at some point. That's helping him to learn on an age and ability appropriate level. It's a matter of building a vocabulary and learning how to spell words that are needed. It's the same with the Sign Language that he's used. There are many concepts in standard Amercan Sign Language that adults use, but children don't. So we have a large dictionary of "Signed English" which has many signs that are more concrete and that children are more likely to use. For example, it has signs for "Incredible Hulk" and "dinosaur" and "monster" which small boys, I think, are more likely to want then many adults.

Just to clarify, his speech is delayed, so signing with the word being said at the same time is another way for him to communicate and learn English.
Also, the older children when they were small would have found the OED hard to handle and confusing to find a particular word without help from an adult. So they also had smaller dictionaries and now I think they each have a Merriam-Webster or the like on their desks for (theoretical) consultation.
Having more then one way to increase knowledge is a Good Thing and different people/ages/abilities etc can find having more then one way to get information very useful.
Ebor
*as may be referred to in an old filksong
"Oh you need teeny tiny eyes
For reading teeny tiny print
Like you need teeny tiny hands
For milking mice"
"...Like you need teeny tiny hooks
For micro-fiche"