So if the Mega Weapon was destroyed in episode 19, and now never existed, then Garmadon wouldn't have awoken the Grundle and the events of episode 18 Childs Play wouldn't have happened, right? So shouldn't Lloyd be a kid again? I'm probably wrong...

okay. let me make this clear for everyone.
i emailed lego.
i will put this in the simplest way possible
mega weapon go boom!
time line restored!
everything go back normal!
Lloyd stay older!
everything as if mega weapon was just হারিয়ে গেছে in a pool of মাখন after Lloyd STILL older

now dont make me talk to আপনি guys like an idiot.
what happened was, when the mega weapon was destroyed, it wasnt erased entirely from ever existing. after it was destroyed, everything went back to normal. thats why Nya was disappearing, she was going back to where show, and everyone and everything should've been in that place and time. the weapons were returned to where they should've been. and then everything played out like it did in season 1
(this পরবর্তি part is a little confusing)
the mega weapon, was sent back to the future (the present time when Garmadon was using it to go back in time) and THEN it was destroyed. the তারকা that was there, well.. we're going to go to science class for a second. so stars are extremely far away from earth (or ninjago). in fact, its so far away, that the light can take a long time to travel back here. (for example, the light আপনি see from the sun right now is from 8 মিনিট ago. meaning the light from the sun takes 8 মিনিট to get from there to earth (and again. অথবা ninjago)) the পরবর্তি closest তারকা to earth, is so far away, that if আপনি used a scale of 1 inch= 100,000 miles. আপনি would have to go (im estimating here) a little under 100 inches away from earth to get to its distance, and the sun would be like 2 inches away. (its really a very hard concept to understand, but its called মহাকাশ for a reason). so, if আপনি were to stand on a তারকা some how, (and remember how far away they are) the light takes so long to travel from earth to that star, that আপনি could be watching the civil war happen right now on that star. yes. আপনি could watch something that happened, a very long time ago, on a তারকা in the বছর 2013. of coarse আপনি would need a very powerful telescope.. but either way, the light takes an insanely long time to get here. and its light. light travels fast.
now then. back to the megaweapon. so when sensei is viewing it in the sky, he's really seeing the light of it from a year, অথবা almost a বছর ago, when it was sent back to the future to be used as it was in the show, but when it was time for 'wrong place wrong time' it was sent out there again and destroyed. so that explains that. now lloyd being older.. well, the mega weapon was still there but when it came to wrong place wrong time, instead of the ninja going to ourourous, and Garmadon seeing lloyd was older, and he using it to go back in time, instead, it was sent back out to the wide area of space, never to be seen again. so that is how it works.
and i emailed lego. thats pretty much what they said.
so please, DROP THE SUBJECT!!!!! ITS A প্রদর্শনী FOR 8 বছর OLDS!! SOMETIMES, UNEXPLAINABLE SHIT HAPPENS! DEAL WITH IT!! (excuse my french ^-^)

actually, since the golden weapons were destroyed when they destroyed the mega weapon, technically that would mean all the ninja would be dead whenever they jump off their flying নৌকা 1000 feet into নিন্‌জাগো city and don't have their vehicles

It may seem odd, but I am really acquainted with the show. The reason that Lloyd didn't turn younger because when the Grundle was awakened with the Mega Weapon, It already happened. When the Ninja destroy the Mega weapon, Lloyd stays in his adult form, because it already happened and they didn't go that far in time.

Sometimes, I hate shows for little kids. They make thing happen like the definition of logic is "Gravy tastes nice."
And I'm not a person to be like that. I have tou friggin know EXACTLEY how something works or, for me, it's ILLOGICAL!!!!!!!!!!!!! That's why I would like ti think of it like this, despite te confusion that নিন্‌জাগো said.

AHEM, now, the mega weapon is composed of the four golden weapons. And at the end of episode 19, the ninja went back in time. (I am currently পাঠ করা a book about time travel, an I'm goin দ্বারা what it said. :3 ) So, দ্বারা definition, time strictly cannot allow two of the same item to be in the exact same time frame together, so automatically, as soon as the mega and golden weapons came into the same time together, they just disappeared. And Lloyd stayed older and everthing else happened as it did.
And please don't tell me anything I missed, because then I will tear my hair out an try to be even আরো logical. :3

I think the Mega Weapon was still in existence.