I am just reminding you that I still need my assignment checked. would be how a native speaker would say this

It's a soft reminder that you are supposed to check my assignment? would be much more clear, so if you wanted to change the minimum amount of wording and have it correct this would be the way to go

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.