My thoughts are in blue, words to delete are in red, suggested rephrasing is in orange.

When 13-year-old Morgan Lane, gifted with a superhuman memory, inherits an ancient, mystical chronometer he owns more than world’s most accurate timepiece. It’s a time machine that can transport him to versions of Earth that exist in timelines before and after our own planet. This got a little stick for me in terms of what that actually means - so he can time travel, but it's also alternate universes? I'd try using that phrasing instead, as I had to sort this one out. But, of course, it’s more than that. It bestows on Morgan another special gift.

Suddenly implanted into Morgan’s mind is a memory and a magical spell forgotten by Princess Leila, a prisoner in Doomguard on a different version of Earth; a spell that, if she could just remember it, will save her life and the world she lives in, one in which dinosaurs and apes have evolved into intelligent, non-human, beings. I don't really understand why the chronometer would not only travel through time in alternate dimensions, and ALSO have a very specific person's memory/spell infused in it. In literally all the times in all the possible worlds, it's got Leila's well being in mind?

Together with Lin Rainbow, his sassy adopted sister from Trinidad who possesses an enchanted amulet, What does the amulet do? What's the power it bestows? Morgan finds himself on an Earth with two moons, where evolved saurians and sapiens are close to war over a precious resource - water. Is this Doomguard, Leila's home? I'd put this paragraph in front of the one before it, identify the dinosaurs and apes here, rather than the above para. Then, illustrate that Morgan has picked up Leila's memory and they need to save her. It will eliminate some of the wording in the para above, which is a bit weighty and convoluted.

Morgan and Lin must save Leila, the way this is written it sounds like the save her first, then do these other things cross the deadly Fleshwarp swamp, find their way through the nightmarish tunnels of Droth and the Soundless Plain to the Forest of Gloom. They face dangers from renegade creatures, mechanical animoids, aerial dragons and slither snakes before reaching the fortress of Doomguard, then rescuing Leila and returning her spell.

But it is just five days till Leila’s execution.

I seek representation for my 42k, middle-grade SF/fantasy Earthscape (first of proposed Timeline Chronicles). It would appeal to young readers of ‘A Wrinkle In Time’, ‘Brightstorm’ and ‘Fablehaven’.

I have been published by HarperCollins and Orion and self-published adult and children’s fiction. I have been long listed for several awards including the Page Turner awards. Great bio!

Overall I think you're in pretty good shape, although I would streamline by changing the order of those two main body paragraphs, giving you space to whittle down the overburdened one.