Do you think it's a good idea to go up to the parents of a girl who just died and tell her it was God's plan?
That sounds horrible! Why do you think it's ok to say that to them?
They need to be able to make peace with it and knowing God decided it will help.
You shouldn't tell people that "it was God's plan." Firstly because it makes it's an insensitive thing to say to grieving people since it makes it sound almost like a good thing that their child died. Secondly, they might not believe in God at all so this message wouldn't be relevant in that case.
I'm not a preacher. I'm just trying to help.
It's good that you are trying to help, but in situations like this, it's usually best just to let the people grieve in their own way. Simply tell them that you are sorry for their loss and let them move through the process of grieving. Although you meant well, your phrasing makes light of their loss which is not a good thing.