Am I right in thinking this is your novel for children, Ian? Have you decided what age group it's for?
You might want to check the openings of recent published novels for whatever age range you've hit on, just to get an idea of how best to start the story. I'm pretty sure that for YA, publishers would want the start to be a lot less info-dumpy and much more in the voice of the child narrator, and I imagine it's the same for novels for younger children. So instead of a rather plain and pedestrian history lesson, it might be an idea to make the beginning a little more dramatic eg (just off the top of my head):
When Grandad was my age, he should have died at the Battle of Whoknowswhere. He'd just been hit by a Whatever, tearing his arm apart -- none of the conscripts had armour of any kind -- and he would have bled to death, for they had no medics or medical supplies out there. But then, out of nowhere, the Stranger appeared, in the middle of the battlefield.
Obviously, the younger age you're going for, the less gory you'd want to make it, but I think the premise still holds good -- you want the reader, however old, to want to read further.
Prose-wise, I noticed a few spelling mistakes eg it should be "enviro
nment" "r
hythm" "Vaugh
an". As and when you type this up, Word or whatever will presumably catch any typos of this kind, but it's always best not to rely on it as it would most likely miss something like the "sometime" you've got which in this context should be "some time" ie two words.
Word would probably also point out the odd missing words and/or incomplete sentences you've got in there eg
"Whether it was the same person or a hologram projected onto each war zone." needs something to finish it off eg "I don't know." Similarly
"Amongst the music Grandad** heard Vivaldi and Ralph Vaughan Williams" doesn't read right even if you're pausing after "music" (which would require a comma) and it really needs something like "were pieces by". As to which line, incidentally, unless the narrator is trained in classical music, I doubt that s/he would be talking of either composer even if the grandfather actually knew the names.
But I'm not sure if Word would also catch the comma splice punctuation errors you're consistently making eg
"I live on a planet that was once called Earth, however as of this year it has been renamed Paradise." -- the comma there isn't strong enough to separate out the two clauses. In that particular case if you used "but" instead of "however" it would be OK, but in the others I noticed, and in this one if you're wedded to "however" (though frankly it doesn't seem a word a child/young person would use) you need to use a semi-colon, or turn the lines into two sentences, or rephrase them altogether.
You've also got some odd wordings that need rethinking eg
"It was then that grass, trees and flowers had grown" -- the pluperfect (aka past perfect) tense of "had grown" doesn't work with "was then" -- the tenses need to be the same ie "was growing" but it still feels a little off since "then" relates to time while the grass etc requires talk of a place. It's best amended to something like "It was then everyone noticed that grass, trees and flowers had grown" which works with the mixed tenses and doesn't require a place.
So, overall, not a bad first effort, but I do think it needs more work both as to the storytelling aspect of how to begin a story and the voice, and as to the grammatical issues surrounding the prose itself.
** It's capital G when referring to him directly as in this line, but lower case g when it's eg "my grandad"