Only one correct answer in regard to the verb here. See:
Potkia expresses continuous kicking that is not temporally confined, whereas potkaista is a momentane verb, expressing one kick lasting only a short, transient moment. [emphasis mine]
Not a native English speaker but, I was under the impression that the present continuous doesn't necessarily mean a repeated action. If I say "my sister is taking a picture of a bug" it doesn't necessarily imply she is taking multiple pictures (though she might), it just means she's doing it at the same time I'm talking about it. Please tell me if I'm wrong or missing something. This has become more important than ever since I started this Finnish course xD
You are right. In some of these comments regarding potkia vs. potkaista, there is some confusion between Finnish frequentative verbs (e.g. lukea vs. lueskella, käydä vs. käyskennellä, or, indeed, potkaista vs. potkia / potkiskella) and the English continuous present tense, but it's important to remember that they are not one and the same thing. Finnish frequentative verbs denote actions that are less temporally or conceptually defined (e.g. lukea = to read, lueskella = to browse or leaf through), whereas the English continuous PT indicates, as you correctly point out, an action that is happening (NB: continuous PT) at the moment the speaker sees it or talks about it. With this in mind, it doesn't matter how many times the kangaroo kicks the car or which verb is used in Finnish – not least because all three potk- verbs above will probably be rendered with English "kick". If the speaker is reporting the scene in real time, the continuous PT is acceptable.