"an only child is an unhappy child" do you agree??

Well, I think it depends on your character
if you are an outgoing child, you may feel unhappy since there is no sibling to play with you or chat with you
but if you are not outgoing, you may be quite satisfied to be an only child since there is no need for you to share your toys or your things to your siblings. Sometimes being an older sibling may need to give up something for your younger sibling, so you may not feel good about it.
So there isn't a definite answer for this question.

No, I don't think the child would be unhappy but lonely.

Being the only child in the family, he/she will always get the attention from the parents. The danger is that the child may get spoiled and get whatever he/she wanted. Also, being the one young kid, it may affect the ability to communicate and work with other people.

However, more positively, the child will have all the loving and caring from the family.

it may be good but it may be not.
if the child is the only child in a family, his/her parents may give more attention to him/ her. His parents may also give me whatever he wants and he don't need to share his things to others.
however, he may be feel bored sometimes as no one plays with him if his parents always need to work.

no,because we can to so many things to forget it!

孤獨的小朋友就是一個傷心的小朋友 ,你同意嗎 ?

pet慍怒,不開心
displeased不開心的,生氣的;被得罪了的