The former child star, 29, had been enjoying a trip to Disneyland with her four-year-old son Luca and decided to share a snippet of their day with her 6.9 million followers.
'Happiest happiest happiest happiest happiest happiest happiest happiest place on earth! We love you @disneyland 🙌🏻❤️🙌🏻❤️🙌🏻❤️🙌🏻❤️🙌🏻❤️🙌🏻❤️🙌🏻❤️🙌🏻' she wrote alongside the picture.
However, almost as soon as the image was posted, Hilary was bombarded with negative comments questioning if the kiss she was giving her son was appropriate.
'I personally feel you should not kiss a child in that way... maybe your husband but not your kid,' a commenter said, whilst another tagged a friend and questioned: 'y is she making out with her son?'
Others said that the photo seemed 'awkward' and 'weird' to them.
'You should not kiss your son like that. There's other ways to show love but not that one. It's confusing for him, it's not healthy...' one added.
It didn't take long for Hilary's fans to hit back at the debate: 'You're clearly not a mother and you're absolutely disgusting if you take this photo in any other way. There is something wrong with you,' one such commenter said.
Another echoed: 'I kiss my babies on the mouth all the time because I love them and that's how I show my affection my goodness people can be so stupid.'
'I honestly see nothing wrong with this photo. Some people need to just mind their own business. She loves her son that's all I see in this photo nothing more and nothing less. @hilaryduff it's a beautiful photo of you and your son!! 😍😍'
Hilary isn't the only celebrity mum to cause controversy with this type of picture. In July of this year, Victoria Beckham shared a photograph of herself kissing daughter Harper on the lips in honour of her birthday, inciting a similar range of comments from her own followers.
The reaction to Victoria's image was so strong that parents began uploading their own pictures of kisses with their children in support of the mum-of-four.
'In support of Victoria Beckham, I feel the world has gone utterly mad, there's no greater love than the love of a parent,' one mother wrote.