@Gold_Scale
I'm partially guessing here, some of the spellings seems to be intentionally wrong since it's supposed to be a kid's sketchbook. My Japanese is also meh at best.
P. 16
ばくねつ(explosive heat) かいじゅう(monster)
(Legit cannot tell what the emphasized characters are supposed to be. Seems like the monster's name.)
Next to the emphasized characters:
いちおくまんとの(A hundred million's? Or it's supposed to be one hundred million tons?)
Last line:
れいとう(freezing) ミサイルに(missile? Really hard to tell what's written here) よわい(weak)。
(On the whole, hard to tell what he's trying to say... weak to freezing missile? Or is the previous line and this one supposed to be one sentence?)
ねせんをは(the text cuts off here)
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P.19, on the cover:
おえかきちょうの(Sketchbook's) そうびやかずる(Equipment drawing?)
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P.21
First row, left:
さばく(Desert) はいおう(はおう/Ruler? There's an extra い if so)
へビヤバー(Probably the name; ヘビ - Snake; ヤバー - probably ヤバイ, dangerous or awesome)
First row, right:
しんちょう(Height) たいじゅう(Weight)
Second row:
3つ(3) の ぶき(Weapons) とたくさんのほそうで(and many armaments)
てきさやっつけてた(defeating enemies?)
(Taken as a whole, seems like using 3 weapons and many armaments to defeat enemies)
Edit:
1. Realized P.21's second row was supposed to be 3つ/三つ - 3, rather than ろつ.
2. Added P.16