It's a parent's worst nightmare, a child goes missing.

For some families, the story can end happily with a reunion, but for others it can mean waiting years for closure.

Today is Missing Children's Day and a family in Sudbury is still waiting, after more than five decades.

Diane Prevost was just two-years-old when she disappeared on Saturday, September 17, 1966, and was never seen again.

Afraid of the water, Diane played in the sand while her father fished on the dock and her siblings enjoyed the lake. Diane asked to return to the trailer at the campsite, which was approximately 500 feet from the beach. Her father indicated that he would take her back, but asked her to wait a moment while he reeled in his line. When he turned to take her back to the campsite, Diane was no longer on the beach. The family began to search for her immediately, and after about an hour, contacted the OPP for assistance. Despite an extensive search of the park and the lake that spanned more than a month, Diane was not found and has never been seen since.

To date, the OPP investigation has not determined the whereabouts of Diane. At the time of her disappearance, Diane was wearing a blue and red checked top with red plaid pants. Diane had a mark on her left knee and a scar on her right foot.

Lise Nastuk is Prevost's sister.

"You know, you tell yourself, oh, we live in Sudbury and these things don't happen. But it does. It does. It does happen." said Nastuk.

Lise was only three-years-old at the time of Diane’s disappearance.

The family was camping at Grundy Lake Provincial Park, an hour south of Sudbury, when Diane vanished without a trace, they believes she was taken.

"There wasn't many rangers in the park and there wasn't that many people. So, they suspected that someone with a car was watching us." said Nastuk.

Life for the Prevost family changed dramatically after that.

"We never went back to a provincial park. We never went to midways or activities like that, where there was lots of people. So, we never enjoyed those things because my parents were just guarding us all the time." said Nastuk.

Lise says she adopted that same vigilance with her own children and even strangers.

"If I see a child that's all alone, it's like, oh my. I go into panic mode." said Nastuk.

Even all these years later, police still have an open file on Diane's disappearance.

Although Lise has never really known her sister, she still believes that Diane is out there somewhere alive.

And while she holds on to that belief, she has a message for other parents.

"To watch your kids! It only just takes a minute. And it happens so fast, and like I said before, you just never know. It can happen to you." said Nastuk.

But to other families with missing children and no real answers, Lise says even if the days turn to months, and the months into years, always keep that hope alive.

Honouring missing children

During the month of May, the Canadian Centre for Child Protection has been sharing "Letters from Home," which are messages from loved ones who are searching for a missing child.

Here is Lise's letter to her sister:

Dear Diane,

It has been so long since we last saw each other. You probably don't remember me but I want you to know that there isn't a day that goes by that I don't think of you.

Fifty-one years ago, someone took you away from us and our lives have never been the same since. Our parents, grand-parents, sister, brother, you and I were camping at Grundy Provincial Park on September 17, 1966 when you disappeared. You wanted to go back to the camper to see our grand-parents, pepère et memère Prévost. You were afraid of the water and didn't want to play in the sand anymore. Mom asked Dad to bring you back and he said he would untangle the fishing line first. You were so headstrong Diane, you didn't wait, why didn't you wait? Someone was waiting to steal you away from us.

To this day, we don't know where you are. Dad never stopped looking for you. For many years, on the anniversary of your disappearance he would go back to Grundy Provincial Park to look for you. He never gave up, even on his dying bed he wasn't ready to go, he had unfinished business, you weren't found. His only wish was to have you back in our family.

Diane, if you read this letter, please know that your real family are still looking for you. You were not given-up for adoption, you were taken from us. We want you to come back home, Diane. We haven't lost hope that you're still waiting for us to find you. Till we meet again, little sister.

Your sister,

Lise

The Canadian Centre and the Ontario Provincial Police are asking everyone to take the time to read and share this letter, in hopes that the Prevost family will be reunited.