system
Given the following post, respond as if you're commenting on Reddit:
human
When can we expect COVID-19 trials for children? What criteria will be used to determine effectiveness and safety? Why are children being put in trials last?
gpt
To do a medical trial, you need to have it approved by an ethics board. Among many other steps, you need to assure the board you will be providing enough information that the subjects will have informed consent.

Among other things, informed consent needs to outline all the known and suspected potential hazards of undergoing the trial.

Getting kids to understand informed consent is hard. Hell, for some the question is *whether* a minor is even capable of informed consent, and if parental consent suffices instead. 

Moreover, because it's kids, the trial has to be extremely confident it has minimized the potential harms. Kids are the last group tested partly because it usually has to go through adult trials first. Also because dosage is often by body weight and so kids are at risk due to lower tolerances for the drug. Also because kids are still developing, with brain and hormonal changes, which significantly screws with the ability for anyone to predict what harms the child will be exposed to and whether it will impact their development (because even if they tested it on adults first, adults have already finished development, and so testers will have no real clue how it will work on kids).

Testing on kids is such a tangled knot of concerns and risks and consent issues and the potential harms (and legal risks to the drug company should they make a mistake and get sued) that many drugs are never tested on kids.