Here, in her own words, is her – and Robin and Colin's – very touching story.
In 1996, I was a stringer for PEOPLE and was sent to Puerto Rico to cover a fundraiser for the American Paralysis Association, headed by Christopher Reeve. Many celebs came down for the event, including Reeve's good friend Robin Williams.
At a beach party the first evening, I introduced myself to him and he couldn't have been more polite. I asked for a 10-to-15 minute interview the next day to discuss the Reeve Foundation, and he said sure.
I then asked if he could possibly spend a few minutes with my 11-year-old son; I told him Colin was often teased about being overweight and his self-esteem could use a boost. Williams visibly brightened and said he'd be happy to.
The next day, Colin and I were having lunch at the hotel; Robin came up to the table and said, "Hi, are you Colin?" You can only imagine how taken aback my son was at a fellow he'd only seen on a movie screen talking to HIM! I excused myself and they sat talking for about 25 minutes. Williams's recent film, Jumanji, was still a hit, and Colin was eager to talk to him about it.
As Colin recalls it, "I just remember he answered every single question I threw at him. He was just as excited as I was talking about his work and his life – I didn't expect that. It was such a great experience in the sense that he actually wanted to talk to me, just some 11-year-old kid. I will never forget the way he made me feel, which was fantastic.
"Then he said, 'So, your mom says you're getting teased in school,' and he proceeded to tell me how HE used to get teased in school when he was a kid and that I shouldn't let it get to me, "because things are going to change. You're going to be great when you get older."
"Later, I was kicking myself because I'd forgotten to get his autograph, but I saw him the next day and I asked him, "Do you remember me?" and he said, "Of course – we just met! And he signed a scrap of paper that said, 'To my new pal, Colin.' "
As Colin's mom, I cannot tell you the difference Robin Williams made in my son's life that day – he will always be a hero to me for the time he took to make one kid feel like a million bucks – and I often wonder if that contributed to Colin going into the acting profession as an improv actor, which he is today in New York.
I only wish I'd written Robin a note in the intervening years to tell him that … of course, I always thought there was time.
And as a side note, he spent nearly half an hour with Colin – and only 10 minutes with me in our interview! I couldn't have been happier.
For more on Robin Williams's tragic death and his legacy of comic genius, pick up the latest issue of PEOPLE, on newsstands Friday
The Williams family is asking well-wishers to send contributions to charities close to the actor's heart in lieu of flowers. Suggested organizations include St. Jude Children's Research Hospital, Challenged Athletes, USO, the Muhammad Ali Parkinson Center, the Christopher and Dana Reeve Foundation and Glide Memorial Church in San Francisco.