The difficulty is understandable, as the use of subjunctive in English is going away over time and is presently inconsistent. Let's clear up your second sentence first:
Pocoyo is pretending that the broom is a guitar.
We don't use the subjunctive here. I suppose we see the fact that Pocoyo is pretending, and that the "that" clause simply explains what he is pretending. Of course, we could look at it the other way, because it isn't a guitar, but either we don't do that anymore or we don't care to go to the trouble. This sounds vaguely Shakespearian:
Pocoyo pretendeth that the broom be a guitar.
Now, look at these:
Pocoyo is acting as if the broom were a guitar.
He needs to act as if he cares for me.
He needs to act as if he were more caring than he is.
You will see that we only use it here with the be verb. Since this verb is the most heavily-inflected in English (probably because it is also the most often-used), the subjunctive forms of it are holding on with more tenacity. This is an archaic construction:
Though he care for me, still must he also thus act.
Now we would say something like "Although he cares for me, he still has to act as if he does" ("act like it" is less formal).
Uses of the subjunctive with verbs other than be remain in fairly specific constructions, generally having to do with wishes, requirements, and the like. Here are some:
It's important that he finish the work by tomorrow.
It's necessary that Pocoyo pretend that the broom is a guitar.
It's desireable that he arrive by four o'clock.
These are more typical of AmE than BrE. The British will typically inject a "should" here: "It's important that he should finish...", for example.
We also use it when expressing a wish (these are sometimes analyzed as imperative rather than subjunctive):
God bless you.
May he win in the end.
God shed his grace on you, and crown your good with brotherhood.
Long live the king. (Long may the king live, may the king live a long time.)
Finally, we have other set phrases where it has remained intact over time:
If it please the court
Until death do us part (until death part us)
Perish the thought
Suffice it to say
We also have a number of these with be, often while we also have similar phrases that no longer use the subjunctive:
If the truth be told
Be that as it may
If need be
Far be it from me
Compare these similar phrases:
If one tells the truth
Whether or not that is so
If there is need
"Far be it from me" is more idiomatic than the other phrases ("far be it from me to do x" means that I emphatically wouldn't do x), so I can't think of a similar phrase that uses the indicative.