these are tough questions to answer off of such little information. there are way to many "what ifs"
do you have any deep dark secrets, skeltons in your closet that the grandparents know about? where you once a stripper, perform in pornos, mass murder, child molester, abusive background....things of this nature. what is there that they can use against you. you are the only one who knows that.
will marrying hurt or help? possibly both. if you are referring to living conditions. marriage definitely looks better than having a live in. it shows a more permanent family structure than a live in. now if he has a history of abuse, violent behavior, criminal record, anything in his past that could be used against him. it's gonna hurt you.
what's there? you know your secrets, you know what they have to use against you.
generally speaking, proving you unfit means that you are not capable of providing for the children. their health and welfare. you are not morally fit to raise them because you lack in good judgement, you can't provide them the nuturing that they need. you forget to feed them occassionally, they are always wearing nasty clothes, you are neglecting their medical needs, there is never any food in your house because you are spending it on crack or marijuana or booze. these are all extreme. it could be something as little as verbal insults. telling the children they are dumb as a box of rocks. belittling them and making them feel worthless.
there are just WAY TO MANY things that could change these.