Firstly, for the choice between "guard" or "watch":
2a: to protect from danger especially by watchful attention : make secure
So if you use this, then it definitely sounds like the robbers are protecting the hostages from some other danger. Therefore "watch" is a better word choice.
The second choice of words really depends on what you want to communicate. Here is how these words would be interpreted if you chose them:
1: MINDFUL, OBSERVANT
To "watch attentively" communicates attention highly focused on the hostages. This is clear, simple, and does not give hint to how the robbers feel.
1a: marked by wary caution or prudence
1b: marked by attentive concern and solicitude
1c: marked by painstaking effort to avoid errors or omissions —often used with of or an infinitive
The key words here are "concern", "caution", and "avoid errors" so the word "careful" carries an understanding of fear. So if robbers are "watching carefully", then it communicates a strong sense of fear in the robbers' minds.
4a: very precise and attentive to details
This has the closest meaning to "attentive" and can be used for a younger audience. But "attentive" is in a higher level of vocabulary, making it a better choice of the two. Like attentive, this does not carry any extra meaning.
1a: paying strict attention : WATCHFUL
The problem with this word is that "watch" is already understood as a part of the meaning. So to "watch observantly" is needlessly repeating the idea of "watching". It sounds almost as bad as "to watch watchfully".
1a: watchful and prompt to meet danger or emergency
Similar to "carefully", this word carries more meaning than simply highly focused. To "watch alertly" conveys that the robbers have been made aware of a danger and have sharpened their attention because of it.