Well I disagree with that.
for one Tolkien isn't written for kids(with the exception of the hobbit which were written for his niece and nephew I believe) And reading his writing style and the sort of concepts he introduces you can see why.
The whole tennant of the LotR is the concept that power corrupts, the ring slowly but surely turned frodo into something he hates and it's will constantly seeks to corrupt, in an attempt return itself to it's true master the Lord of the Rings(Sauron) Namely Galdriel(wielder of the ring of air) Boromier, Faramier, Gandalf(owner of the ring of fire, I bet you wondered how he could go one on one with a big fiery balrog and suffer no burns?) Any of the people of power who came into contact with; Saurons other half(don't be fooled the ring has a will of it's own and much of Saurons power is invested in it, it is almost sentient) And the book outlines the sort of mental torture the ring exhibits in those not meant to wield it(i.e. anyone but Sauron) Gollums split personality is far more interestingly played out in the book than in the film, and he oddly is quite a deep character for a bit player.
I think it's very difficult to have true depth in so many characters, but Sam and Frodo's relationship is interesting, the loyal but lower class friend who loves Frodo deeply. Also Gimli and Legolas, elves and dwarves being traditional enemies from way back when. Aragorn is meant to be aloof and distant, he carries the torture of the weakness of Isildur and his shame is a constant torture to him. Gandalf is equally interesting a man who came to middle Earth at the beginning of the second age and failed his first mission on middle Earth, to defend Arnor and Gondor(originally one kingdom under men, until Isuldurs line was broken Arnor was destroyed by the witchking, the head ring wraith and his genius for wars of attrition.
Pippin and merry are comic characters, who's friendship is an interesting diversion, untill they part. Eowyn the only strong female character is interesting, a woman in a mans world, struggling to achieve the recognition of her borthers, who eventually does so by destroying the Witch king.
No man can kill me.
I am no man as she rams a sword through his helmet thus after Pippin knocked the crown from his head, or one of the nine rings for the mortal lords and killing him.
Elrond is interesting(owner of the ring of water) as shown by his control of water at the ford where the nine ring watiths are slain. Traditionally the Elves have always been reluctant to get involved in the wars of men, and his reticence is born of pain and history, it is hard to know his motivations though as the immortal of the elves are governed by fate and they felt the pull of the west so strongly when they left it.
Saruman is an example of how power corrupts originally the mightiest of the Istari or mages(Maiar and immensley powered individuals of which only Sauron the black(the mightiest of all) Saruman the white the mighteist of those who oppose him and Radaghast the brown have any real part in the book) Sarumans quest for power to oppose Sauron leads him to look into using the palantiri, seeing stones that enable communication over great distances and also can show the future. Sauron feeds and corrupts his search for power and leads him to turn against the side of good.
Borromier tortured by the faith his father placed in him to free his people, but after years of war with Sauron is honestly seeking something to turn the tide against the dark lord. He like all men is eventually corrupted by the ring and it's influence leads to his death. The ring looks to pass back to Sauron via a man the most easily corrupted of middle Earths races.
But yeah feel free to call it a kids book and to claim it has no depth.