This reminds me of this one episode of Corner Gas. It's was a small part of the episode, saying how you can manipulate children to listen to you:
1. Use their full name (they aren't used to hearing it, so it surprises them)
2. <insert chore here> (eg. go to bed)
3. If you don't <insert chore here>, you don't want to know what will happen (as in a "what's in store for you" kinda taunt)
And it goes to say that the last bit is particularly effective because a child's imagination is so overly hyper active that they will picture the worst possible thing (like a vampire, or some other mythical being).
Okay so, about the poem. This is one scary nightmare. Personally, I don't think I have ever had a fantastical nightmare or anything like that, like this one. (Usually my nightmares are either about murderers or social issues o_O)
Because of the descriptions you have used, it is really up to the reader to picture these monsters (not saying that it is otherwise in any poetry), but depending on your knowledge of ghouls and creepiness, it varies, I guess.
One tiny thing I would change:
"The wasp are planting spiders in the yard"
"The wasps are planting spiders in the yard"
Haha, and the reference to family is great because it's relatable in that the older brother is always trying to freak out the younger sibling (or an older sister...) and it's a scene that most of us are familiar with, be it because of real life, or in movies/TV shows and such.
And the last line... ties the whole thing up nicely. We know that this kid will (hopefully?) be saved by the almighty mother figure. And as said somewhere by someone, 'mum's the word!'
Anyway, this is a really creative and fantastical poem, I haven't read anything like this since... since ages. D;