Native English speakers use "gentle reminder," not "soft reminder" or "kind reminder." These last two are not normal native expressions.
So, to express your sentence the way a native speaker would, you can say:
This is a gentle reminder that you are supposed to check my homework.
On the other hand, when you tell someone that they are "supposed to do something," that is usually not a gentle reminder. So it might be better to say:
This is a gentle reminder to check my homework.