Explanations

CEREAL= /sɪərɪəl/ As you can see, the pronunciation of the word "cereal" and "serial" is exactly the same. A "serial killer" is a person who kills many people one after the other.
MUMMIES= See picture.
KEEPING FRIENDS= If you keep friends, you continue to be their friends. If you can't keep friends, you may make friends but you soon lose them because you don't know how to be a good friend or keep a relationship.
WRAPPED UP IN THEMSELVES= If you are wrapped up in yourself, you are selfish and you only think of yourself, not of others. But the word WRAPPED also means to cover a present or something usually using paper or cloth. To WRAP something Up is to cover something using paper or cloth. Mummies are wrapped in cloth strips, so of course, they're always very much wrapped up in themselves.
DEAD ENDS= A little street or alley which has one entry but no exit (also called "a cul-de-sack"). Also a road which is cut, blocked or unfinished, so it suddenly stops and you can't continue.
CHEWSDAY= To chew is to use your teeth to smash food before you shallow it. But in colloquial English, the day Tuesday is usually pronounced /tʃu:zdeɪ/ , so "chews-day" and "Tuesday" sound exactly the same.
Remember the most important assimilation rule in English: T+Y=CH (/t/+/j/=/tʃ/), so /tju:zdeɪ >> /tʃu:zdeɪ/.
COLLECTED= A you are collected you keep calm and don't lose control even in a shocking situation. If you lose control because of a strong emotion (fear, excitement, happiness, nerves) people may tell you "please, collect yourself".
GETS UNDER THEIR SKIN= If something gets under your skin, it's really irritating. Of course skeletons have no skin. (SKIN is the outer cover of our body).
BREED= A breed of horses (also dogs or any other domestic animal) is a genetic family of horses which share the same characteristics because they have common ancestors. For instance, the Arabic horse and the percheron are both kinds of horses, but they look different and have different features, so they are two different breeds.
BOOGIE MAN= A monster that can get to you through your dreams, especially through your nightmares.
NIGHT MARES= A nightmare is a bad dream, but this word looks like a composition of two other words: night + mares. The second part of the word MARE is also a female horse.
WRAP MUSIC= As we saw before, mummies are wrapped in cloth straps. The pronunciation of the words WRAP and RAP is exactly the same, so "wrap music" sounds like "rap music".
FALLING FOR…= If you fall for a trick, they can trick you (deceive you, fool you) because you are not clever enough to know that it's a trick.
SUCKER= To suck is to draw something by producing a vacuum with your mouth. Vampires suck blood, so they are "blood suckers". But the word A SUCKER usually means a person who is a bit silly and very easy to trick. So if they play many tricks on you and you fall for them very often, you are a sucker.
STUNT= A stunt man is a man who is big strong and well developed and usually brave (or even brute).
GUTS= The part of your digestive system which is like a tube and connects your stomach with the end of your body. A common expression says that if you HAVE GUTS, you are brave. Skeletons, of course, have no guts (so they are cowards)
WIZARD= A man who practises witchcraft, black magic. The male variety of "witch".
SPELL CHECKER= The spell checker is an application in your computer that checks your writing and shows all the spelling mistakes (for example the programme "Word", from the suit "Microsoft Office" has a spell checker included, so it can automatically correct your spelling). But the word SPELL is also an enchantment, a word or formula believed to have magic power.
BROOM= A tool to sweep the floor (see picture)
A CLEAN SWEEP= is an improvement, an embellishment, so if you want "a clean sweep" you want to look better (cleaning your dirty old broom will help if you are a witch). But the literal meaning of this common expression is the action of completely cleaning an area by sweeping it carefully (using a broom).