Little Baby Bum, founded by London couple Derek and Cannis Holder in 2011, has been bought by a firm called Moonbug, according to reports.
The couple's 54-minute animated wheels on the bus video has been viewed more than two billion times – the 20th most popular video in Youtube's history.
Featuring long videos with 3D characters and nursery rhymes, Little Baby Bum is the site's ninth most watched channel.
The amount paid has not been disclosed but analysts estimated it could be between £6m and £8m based on other valuations of Youtube channels.
Moonbug was founded by a trio of industry stalwarts, Rene Rechtman, John Robson and Alfred Chubb.
Rechtman was formerly president of Youtube network Maker Studios, which was sold to Disney for more than $650m (£497m) in 2014.
Robson is managing director of children's entertainment content creator WildBrain, an arm of Ben 10 and Teletubbies controller DHX Media.
Little Baby Bum's public records showed that Rechtman, Chubb and Robson became directors in July - the same month Derek and Cannis Holder left the company.
Derek Holder told Bloomberg: “We had come to a crossroads to take the company to the next level.
“To get from that to what really needs to happen now for the brand is that it needs to have a proper team to upscale it.”
He said the new owners would try to take the brand to TV and increase Youtube output from two videos per week to four or five.