His channel was launched in March 2015 when Ryan was three, and it now has more than 10 million followers.
Ryan's mother Loann quit her job as a high school science teacher to work on the channel full-time, and it's a move that's paid off.
In an interview with TubeFilter, Ryan's mother explained how the phenomenon started.
"Ryan was watching a lot of toy review channels — some of his favourites are EvanTubeHD and Hulyan Maya — because they used to make a lot of videos about Thomas the Tank Engine, and Ryan was super into Thomas," she said.
"One day, he asked me, 'How come I'm not on YouTube when all the other kids are?' So we just decided — yeah, we can do that. Then, we took him to the store to get his very first toy — I think it was a Lego train set — and it all started from there."
Ryan's channel took off after a video of him opening and reviewing a Giant Egg Surprise went viral.
Since then, he has posted daily reviews of new toys and foods for kids.
Loann and husband Sean, a structural engineer, are putting the YouTube earnings into a college savings account for Ryan and a trust fund for him and his twin sisters, Emma and Kate.
While the family have struck gold with the channel, Sean and Loann say the family will only continue making the videos until Ryan gets bored with them.
So could your child become the next YouTube sensation?
YouTube expert Josh Cohen told The Verge there were more lucrative opportunities just waiting to be taken.
"It's the biggest of this genre of programming that is getting billions of views a week on YouTube. Really nobody is talking about it, but it's crazy once you start scratching the surface," he said.