Maybe they dont have the ability to unpack it to the same extent you do, but pretty confident a 12 year old can understand the basics of "not all points are equal in weight attributed to them" and can grasp the concept that certain negatives can outweigh benefits.
The question does not ask them to justify colonialism.
They also haven't been given this in a vacuum presumably, and have had primary sources and class discussions led by the teacher prior which prompts them to think about these things.
I actually remember my year 7 and year 8 history lessons precisely for this reason because I loved how they encouraged this way fot hinking that I hadn't done in primary school. Obviously by adult standards it was very basic, but it felt ground breaking and eye opening at the time.
Give 12 year olds a bit more credit
I am very inclined not to, because 'Obviously by adult standards it was very basic' is a predictable and massive problem.
The risk of pitching this question, worded as it is, at children of 12 is that this is as deeply as they are ever going to be led into this question, and the wording implies some balancing out.
I am not worried that the question invites them to justify colonialism. My concern is that they are asked essentially to come up with a value judgement, whether they realise it or not. One regrettable aspect of that is that any conclusion they arrive at will be their fundamental assumption about colonialism for years to come. It is not the only problem, however.
12 is a delicate age. Students of 12 should be confined to exposure to primary sources, chronology, and commentary on the primary sources, perhaps, if a teacher wants to go into historiography, in comparison with primary source material and secondary source assessments of more recent times. Essay questions of the sort that the OP has posted are not only completely inappropriate intellectual tasks for tweens, they are completely inappropriate tasks for any student of History.
At 12, they should be invited to reflect on attitudes of the era being studied, in their own context, in order to fully understand each time period.
If history is to be taught as an opportunity to indulge in value judgements (which is not what the study of history is supposed to be) the danger is that it is presented as a monument of progress, with the current generation at the apex, with absolutely nothing learned (in terms of analytic skills or pattern recognition) that is relevant to dissecting the times we ourselves live in.
To sum up -
A History curriculum should not be treated as an opportunity to allow students to proceed to value judgements or for teachers to lead students to value judgements.
If nothing else, the study of History shows the dangers inherent in using the values of any given current day to pass judgement on any time in the past.