No, you can't do this. Change Shape lets a dragon turn into "a humanoid or beast that has a challenge rating no higher than its own".
A level 20 wizard doesn't have a challenge rating. You could turn into the CR 12 Archmage, if you wanted, but you couldn't become any player character, because player characters don't have challenge ratings.
The wiki page you're reading is probably wrong. It doesn't cite a source for its information. There aren't any sources in 5th edition D&D that help us find the answer (the Monster Manual has nothing of note about silver dragon diets), so I've dug through older fluff to get a picture of what a silver dragon eats:
The Draconomicon, a 3.5 sourcebook, has this to say about dragon diet on page 10:
Dragons are carnivores and top predators, though in practice they are omnivorous
and eat almost anything if necessary. A dragon can literally eat rock or dirt and survive. Some dragons, particularly the metallic ones, subsist primarily on inorganic fare. Such dining habits, however, are cultural in origin.
Unfortunately for a dragon’s neighbors, the difference between how much a dragon
must eat and how much it is able to eat is vast. Most dragons can easily consume half their own weight in meat every day, and many gladly do so if sufficient prey is available. Even after habitual gorging, a dragon seldom gets fat. Instead, it converts its food into elemental energy and stores it for later use. Much of this stored energy is expended on breath weapons and on the numerous growth spurts (see below) that a dragon experiences throughout its life
In 4th edition, they expanded the information slightly. In Draconomicon: Chromatic Dragons, the diet of a dragon is elaborated on page 12, noting that while they can eat anything, minerals, dirt, and the like are not very nutritious, so it has to eat a lot more than normal if it subsists entirely on inorganic material ("at least as much as its own body weight per day to maintain health"). If a dragon is eating actual organic food, particularly meat, then it needs to eat about 15-20% of its body weight per day to stay healthy and keep growing. It also says that in order for a dragon to become overweight, it has to eat a significantly larger amount of food (with an anecdote about an obese dragon that ate twice its body weight in food each day).
In Draconomicon: Metallic Dragons, it's also stated that eating the metal of a dragon's type helps promote scale growth and regrowth.
Also, while it doesn't have much about the feeding of baby dragons, I would greatly recommend trying to get your hands on Dragon Magazine #320, for the article on page 46, "Dragons: The Perilous Burden." It's got a details on the mindset of a young dragon and what they normally experience when fostered with humanoids.
Best Answer
The plan should not work due to the word game being played
You are inflicting something like progeria on the young dragons
Growth and maturation, and aging, are not direct synonyms. (Example: I have never stopped increasing in age, but I stopped growing taller in my mid to late teens, and I was full grown/developed somewhere in my early twenties).
The connotation of "growth" is positive - there's more to it than a numerical ticker. In the description of ghost's ability, "aging" is negative effect. It is a debilitating effect. (see below)
It takes positive energy, greater restoration, to reverse the negative magical effect of aging.
If the BBEG tries to age baby dragons with a ghost, you could expect them to die prematurely (life expectancy for progeria is somewhere in one's teens) rather than to grow into their next phase of maturity. Growing into maturity is a gradual process that over time (and by eating a lot of Pyrena's Red Dragon Chow, or someone's livestock) sees the dragon develop into it's next size and power level.
Why can't the ghost's aging be beneficial?
Firstly, because it is overcome by the greater restoration spell.
And, per the ghost ability's description, ghost-inflicted aging if cast within 24 hours.
Secondly, consider the basic premise of who is adventuring in D&D 5e (and thus who will be aged by monsters such as a ghost): it isn't children, it is player characters. Player characters are presumed to be mature members of their race/species, from young adults to full fledged adults, and possibly old adults, who got the urge to adventure1 (Sage background, for example).
The PCs have stopped growing and maturing, thus a ghost aging them (in the negative sense) accelerates their approach to death and / or disability (aside from a few class/sub class features like the Druid's timeless body).
Aren't exploits part of the fun of D&D? Sure they are.
Since you are the DM, you can do as you like and are not bound by any RAW in the first place. (Such that I wonder why you asked the question, but thanks for asking since it got me thinking! 😃 ) What you have proposed is an overly-complicated means to an end, which is to throw a "gotcha" at your players when they are out saving the kingdom / region / villages from a bunch of marauding young dragons. (A common enough malady in a D&D campaign).
Review your purpose in doing that:
What plans do you have to provide a form of subtle cuing that "something is not right" before the on / off switch of "dead dragon reverts to the form of child" hits them in the face? See the Alexandrian's "three clue rule" for some guidance on that.
@CaptainMan made this suggestion, for a clue on weird magical things going on: at some point the party finds "old baby dragon" skeletons in an evil wizard's lair that were the result of an attempt at this scheme
If the BBEG can cast 9th level spells, is this solution the best use of his / her resources? Granted, this scheme you propose fully fits into the cackling evil villain trope; BBEG's being purely evil is very much expected. @Rubiksmoose offers a more efficient alternative, though it may not give you the horror element that you are looking for.
While you can do as you wish, this answer is intended to provide an alternate view of both growth and aging than as game mechanics to exploit, and recommends going into a little more depth in providing your players a challenge, however horrific you choose to make it.
The other contributor to this answer is the points made by the devs for this edition regarding the default to plain English interpretations of the rules text.
1 (Backgrounds, PHB, pp. 127-141, describe most professions as what the character has done 'for years' before choosing a life of adventure; Guild Artisan, Soldier, Sailor and Sage in particular)