"A mi hijo" and not "Mi hijo" ?
My son likes the crib = A mi hijo le gusta la cuna
Why is A used in the beginnging ("A mi hijo") and why is "Mi hijo" not an acceptable answer?
The verb "gustar" as you probably already know means "to please". In english we say "My son likes the crib", but in spanish we say something more like, "The crib pleases my son". In spanish, whenever something happens to a person we use "to", or "a", often called "the personal a". If you notice the little "le" there, that means the same thing as saying in this case "to my son", so putting "a mi hijo" in the front is really a redundancy, but makes the sentence way more clear. Without "a mi hijo" the reader/listener would only read/hear "le gusta la cuna", in which case they would know the crib pleases someone or something, but have no idea what exactly, therefore they specify, "a mi hijo". The "le" is left there because it's still proper grammer and sounds really funny without it. Hope that helps.
The use of 'a' in this case is not the personal a. It means "to" as poffandi said (literally: to my son the crib is pleasing = the crib is pleasing to my son).
However, the personal a is something different. It is used when a person is the direct object of an action. For example, "The girl ate the lion" = "la niña comió el león" but "The lion ate the girl" = "el león comió a la niña". The 'a' before la niña is the personal a.