ha suki desu vs ga suki desu

Recently, a Japanese pen pal wrote 'watashi mo ...ha daisuki da yo' to me.

I've always learned that when talking about your zukikirai, you should use 'ga'.

Also, when I went to the movies in Japan to see Harry Potter once, there was a part where one of the evil characters said 'I hate children' which in Japanese was subtitled as 'kodomo ha (dai)kirai desu'

I've kept that Harry Potter sentence in the back of my mind ever since, and since my penpal wrote the above sentence to me the other day, it led me to ask this question on the forum.

So could I please get a clearer explanation as to why sometimes 'ha' is used instead of 'ga'?

I am familiar with the sentence pattern 'A ha suki desu ga B ha suki de ha nai', however that only applies when one is making a comparison between two things, which isn't the case in the above examples I have given (at least I think it isn't)

Many thanks

One way to look at it:

子どもはきらいです。
As for kids, I hate them.
子どもがきらいです。
Kids are what I hate.

Keep in mind that は can also act as a contrastive even for a single action. This is the same は you see in ではない / じゃない.

これは本ではない。
As for this, a book is not what it is.
This is not a book (but it might be something else).

Or more complex, with adjectives:
あつい→あつくない→あつくはない
hot→not hot→hot is what it is not

And verbs (negative):
食べる→食べない→[食べ＋は＋する（しない）]→食べはしない
Eat→Not eat→Eat is not what [I] do
(maybe you can drink it)