From the dictionary, I know that It's a good thing is used when something happened, it is lucky that it happened.
But in the following sentence, that is not describing something has happened and instead it is just about a future situation or an opinion. So I think it is inappropriate to use this clause - It's a good thing.
If just saying It's good seems not correct, then what should be used here?
It is a good thing that if grandparents can contribute their efforts to care for their grandchildren.