もれなく
=漏れなく、漏れることなく、例外なく
=without any exceptions
応募したら
もれなくもらえる景品
=応募したら、応募したヒト
全員に必ずもらえる景品
お菓子くれないと悪戯
もれなくしちゃうぜ
=お菓子をくれないなら、いたずらを
お菓子をくれなかったヒト全員に必ずしますからね。例外はありませんよ。お菓子をくれないのなら、いたずらを免れることはありませんからね。だからお菓子を必ずくださいね!
=お菓子をくれないと、
必ず悪戯をしちゃうぜ。
they will play tricks on every person who's not giving candy to them
If you don't give the Haloween candies to us, we'll play tricks on all of you for sure. Keep in mind that there will be no exceptions!
We will play tricks on all of those who do not give us candies. No exceptions!
Another interpretation:
お菓子くれないと悪戯
もれなくしちゃうぜ
=お菓子くれないと
あらゆる種類の悪戯を例外なくしちゃうぜ
=お菓子くれないと
悪戯を全部しちゃうぜ
=お菓子くれないと
悪戯を考えうる種類の中の全部しちゃうぜ
I think this iterpretation is much less likely somehow, although it might be possible technically.
I wonder why, and this is my assumption:
"Trick or treat" is a kind of rhetorical sentence. They have no intention to actually play tricks on adults. They only request to give them "treat." Therefore, they would not think about the concrete plans how to trick adluts.
Therefore, もれなく is less likely to modify "every kind of tricks."
Another reason comes from grammatical consideration:
もれなくもらえる is a kind of set phrase; the usage always midifies people or "participants", like "all the people/participants" "every people/participants".
もれなく usually doesn't modify the variety of the object 景品, present, or tricks, like "every tricks," "all the tricks that exist on this planet" or something.
However, this sentence itself is a quite colloquial sentence, and might not be a "standard" Japanese sentence. So it's a confusing one. We need to ask the speaker themselves in order to make it clear.